Some of them are partial abstinence, some are full abstinence, and some are none of the above!
I think part of the reason for the difference is that some of the Holy Days are worldwide (i.e. Christmas, Ascension), while some are country specific (i.e. Immaculate Conception, All Saints). The country-specific holy days are made a holy day by the bishops of the country (i.e. St Patrick is a holy day in Ireland; Ephiphany is a holy day in much of South America and probably the Middle East). I would think the vigils which are world-wide would have a full fast/abstinence and they would also be the oldest holy days of our Faith (i.e. Vigils of Christmas, Easter, Pentecost) and these feasts also USED TO have octaves of celebration afterwards. So many, the full fast/abstinence is related to preparing for the octave of celebration? The feasts of All Saints, All Souls and the Immaculate Conception are relatively new feasts, compared to Christmas, Easter and Pentecost.
Also, one wonders why the feast of the Annunciation is not a holy day? I think it was Anne Catherine Emerich who explained the marvelous importance of March 25:
1. The sixth day of creation, when God created Adam
2. The actual day of the Annunciation
3. The day of Good Friday
4. ...I think there was one more connection to the Old Testament, either Abraham or Moses...
5. Dante picked this day to begin his 'Divine Comedy' story
6. Tolkien picked this day specificially as the day when the One Ring was destroyed (obvious parallel to Good Friday)
Anyway, the Church knows best and things happen in God's time but I wouldn't be surprised if this was made a Holy Day in the future.