No. Buying Crysta now only adds Crysta to your Crysta balance. The Crysta to cover FFXIV game time will not be deducted from your Crysta balance until you run out of game time. Vet rewards activate when you pay for FFXIV game time. Crysta can (theoretically) be used for things other than FFXIV game time, so simply having a Crysta balance doesn't cut it.