If the will provides for the gift over to Y’s issue to occur “if Y fails to obtain a vested interest”, to my mind it would suggest that there is another requirement than merely surviving the testator. It could be as straightforward as there being a stipulation that any beneficiary must survive the testator by 30 days, buried somewhere in the administrative provisions.
The answer as to when Y’s interest in remainder vests absolutely will depend upon the precise wording of the will.
It may be worthwhile looking at the original will instructions (if they are still available) as, following the Supreme Court decision in Marley v. Rawlings, my understanding is that the intention is paramount regardless of the wording of the will.
With regard to deprivation, I believe a gift of any interest in remainder to which Y is entitled would be classified as “deprivation”. The question is probably whether the relevant authority would recognise it as such.
I believe there is still a market for reversionary interests, and it may be that any deprivation would be valued by reference to the potential sale value of the interest, rather than the value of the trust fund as at the date of gift. However, if Y might not be expected to survive X, there could be an argument that the interest in reversion has no value to Y during their lifetime and therefore a nil value should be ascribed to the gift for deprivation purposes.