From what I understand, the sub account does not gain PS+, even if you subscribed to it on the master account. This isn't necessarily a problem with the PS3/4, as you can download things from the master account and play them with the sub account, however if you used the sub account on the Vita you will not be able to download and play any games from the master account, as the Vita doesn't allow multiple PSN accounts at the same time.
Therefore, you should use the PS+ subscription on a master account that you must use on the Vita, whereas you can keep the sub account for the PS3/4.
EDIT: It should be noted that you can disable all restrictions on the sub account and purchase a PS+ subscription via Sony's store online, however PS+ Time Cards will not work.