Why would that aspect of it have changed?  Being a Vigil, it's transitional into the Feast itself.  During the Easter Vigil itself, there is no Mass.  Now, Easter Mass typically follows close behind it, if the Vigil itself is celebrated near midnight, but they're distinct things.  Even in the pre-Pius XII version, the Vigil itself was held on Holy Saturday morning, followed by the Mass of the Resurrection ... whereupon Trads everywhere greet one another with "Happy Easter" on Holy Saturday, as if Christ had risen on the Second Day, rather than the Third.  Timing of the Easter Vigil was in fact a nonsensical abuse and the Pius XII move back to its proper time was quite welcome.  In fact, if you look at his text introducing the changes, he's referring almost exclusively to the time (and the accompying fasting changes that are necessary if you're going to have evening Masses now, e.g. for Holy Thurday).  I'm not convinced that Pius XII had much of anything to do with the reform of the Rite, since he suffered a catastrophic health collapse in 1954, and many of those around him testified that he was not running things from about that time until the end of his pontificate.  It would be as if Bergoglio in his current state of health were behind some major "reform" of the NO, where you'd know that he had precious little to do with it, given that he can barely function.

Why would that aspect of it have changed?  Being a Vigil, it's transitional into the Feast itself.  During the Easter Vigil itself, there is no Mass.  Now, Easter Mass typically follows close behind it, if the Vigil itself is celebrated near midnight, but they're distinct things.  Even in the pre-Pius XII version, the Vigil itself was held on Holy Saturday morning, followed by the Mass of the Resurrection ... whereupon Trads everywhere greet one another with "Happy Easter" on Holy Saturday, as if Christ had risen on the Second Day, rather than the Third.  Timing of the Easter Vigil was in fact a nonsensical abuse and the Pius XII move back to its proper time was quite welcome.  In fact, if you look at his text introducing the changes, he's referring almost exclusively to the time (and the accompying fasting changes that are necessary if you're going to have evening Masses now, e.g. for Holy Thurday).  I'm not convinced that Pius XII had much of anything to do with the reform of the Rite, since he suffered a catastrophic health collapse in 1954, and many of those around him testified that he was not running things from about that time until the end of his pontificate.  It would be as if Bergoglio in his current state of health were behind some major "reform" of the NO, where you'd know that he had precious little to do with it, given that he can barely function.

Why would the mass portion not be considered part of the vigil? All the other major vigils such as Pentecost, Christmas, and Sts Peter & Paul have a mass that occurs in the morning. 

Either way I'm mostly asking because I can not attend a pre 55 Easter Vigil this year and will instead be going to a Holy Saturday Divine Liturgy in the morning and a Pius XII reformed Vigil in the evening. 

Another question would be did Pius XII see the reformed Vigil as fulfilling the Sunday obligation if the mass portion began after midnight? What happens is the Vigil ended slightly before midnight?

It has always seemed to me that on the Easter Vigil we are awaiting the Resurrection of Our Lord and we are celebrating the mysteries of His descent into hades to conquer death and free the old testament saints.

You're still conflating the Vigil with the Easter Mass.  Neither before nor after Pope Pius XII did the Vigil satisfy Sunday obligation, since it's not a Mass.  Easter Mass itself needed to have at least the Offertory begin after midnight to be considered as satisfying the Sunday obligation.

In the early Church the Vigil lasted the entire night, with the Easter Mass (or Mass of the Feast on other Feast Days) beginning at sunrise.  It has nothing to do with Our Lord's descent into Hades per se, as every Major Feast had a vigil.  Catechumens were typically baptized just prior to the Easter Mass, and the many lessons and psalms were intended as an at-least symbolic conclusion of their instruction in the faith, and also for everyone else basically detailed in outline all of salvation history culminating in the Resurrection.

Why would the mass portion not be considered part of the vigil? All the other major vigils such as Pentecost, Christmas, and Sts Peter & Paul have a mass that occurs in the morning.

Either way I'm mostly asking because I can not attend a pre 55 Easter Vigil this year and will instead be going to a Holy Saturday Divine Liturgy in the morning and a Pius XII reformed Vigil in the evening.

What's required is attendance at Easter Mass on Easter Sunday.  If there's a Mass AFTER the actual Vigil, then in the Traditional rules, the Offertory must begin after midnight for it to count for the Sunday / Easter obligation.  It has absolutely nothing to do with pre-1955 of post-1955.  Neither in pre- nor in post- 1955 did the Mass count for your Easter Sunday obligatoin except under those conditions.  It was only in the Novus Ordo era that something before midnight would count toward your Easter Sunday obligation.

Pre-1955:  Easter Vigil on Satrurday Morning.  Easter Mass on Saturday Morning.  -- does not satisfy obligation for Easter Sunday

Post-1955:  Easter Vigil on Staruday Evening.  Easter Mass on Easter Sunday Morning (with Offertory after Mass) -- Easter Sunday Mass satisfies Easter Sunday obligation.

Post-1955: Easter Vigil on Saturday Evening.  Easter Mass on Saturday Evening. -- does not satisfy obligtation for Easter Sunday

Novus Ordo:  Easter Vigil on Saturday Evening.  Easter Mass on Saturday Evening. -- does satisfy the obligation (according to them).

https://www.catholicculture.org/culture/library/view.cfm?recnum=11136

1951 just dealt with time changes.  And then the Sacred Congregation of Rites here say that Pius XII gave them some kind of "mandate", and in terms of what Pius XII approved, it depends upon what the Congregation actually told them was being done.  There's almost nothing in all this verbiage about detailed changes within the rites themselves, and it's all about the times.

https://www.catholicculture.org/culture/library/view.cfm?recnum=11136

1951 just dealt with time changes. 

The link is for the 1955 decree Maxima Redemptionis.

The 1951 decree was titled Dominicae Resurrectionis vigiliam and is published in AAS 43 (1951) pp. 128-129.  The rubrics and texts for the new service continue after the decree pp. 130-137.

The form of service only lasted for a year with a second version appearing in 1952, itself replaced my the texts and rubrics published along with Maxima Redemptionis for 1956 and subsequent years.

Are you saying that the Vigil of Christmas that happens on the morning of Christmas Eve, in violet vestments, is already the Mass of Christmas?

I'm clearly saying the opposite, that the "Vigil" Mass of the day before Christmas is not Christmas Mass.

You are very confused.  Mass on the day before Christmas isn't actually the Vigil in the same sense as the Easter Vigil, but was merely the Mass for the day before Christmas.  One of the few other "Vigils" that was retained through pre-1955 to compare it to is that of Pentecost.  Rest of them slowly fell into disuse and were no longer true Vigils.

For the last time, the Vigils themselves were NOT the actual Masses of the feast, but, as the name implies, a set of services, consisting of sung Psalms, readings, etc. that went through the night in anticipation of and leading up to the actual Mass of the Feast itself.  That's what the term "vigil" actually means in Latin, a night-watch.  So they stayed up all night in anticipation of the Feast itself.  If today there's a "Mass" on the "Vigil" of Christmas (where the term had just loosely been transferred to mean "the Day Before"), that's just because the Vigil itself felt into disuse and was no longer celebrated as a true Vigil.