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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.