If you want to get a good idea of what has been lost, look at the entry here on the St Lawrence Press blog which goes through what the Vigil of Pentecost used to consist of. You will see how it echoes the Easter Vigil, not least in the way in which they stress Baptism.
It is important to stress that the loss of this celebration has nothing to do with Pope John's Missal or with Vatican II: it was suppressed by Pius XII at the same time as the reordered the ceremonies of Holy Week. Not only was the shape and direction of Holy Week changed, but Pentecost was reduced.
As I mentioned recently, it is clear that the change movement was active a lot earlier than I had realised. Another throwaway line in the 1939 hand Missal comes after a reminder that the Vigil originally took place at night: "It is this which must be kept in mind in order to understand all the offices this morning". Well, no, actually.
This sort of archaeologism is wrong for two reasons: first, because it supposes that up to 1955 nobody except for a tiny handful of scholars actually understood what was going on; second, because it is so selective. When Pius XII reordered Holy Week, I bet it never entered his head or his advisers' that perhaps he should, for example, reintroduce the fasting practices which characterised Holy Week in the fourth century and which shaped the liturgical experience for those who observed the original late evening and night time vigil. (Actually, it's wrong for lots more reasons, but these are the two I want to stress here.)
Why had the Ester Vigil ended up being celebrated on the morning of Holy Saturday, while the Vigil of Pentecost took place after None, ie in the afternoon or early evening of Whitsun eve? I don't know the answer, but it demonstrates that the organic development of the liturgy does not depend on a fiat from a Vatican liturgical expert which would aim, as we came to see in Bugnini's day, at flat standardisation, but on gradual changes arising from the nature and importance of a particular part of the celebration of the liturgy.
By the time the major revisions to the liturgy which culminated in the 1967 Novus Ordo were being studied, the abolition of even the Octave of Pentecost went through pretty well on the nod. Why such an important feast was so downgraded is something I don't understand at all.