Sony should consider some sort of family package for plus subscriptions. With the Vita getting Plus next month it would be nice to get 2-4 subscriptions at a discounted price. I preordered the Assassins Creed Vita bundle for my fiance but would probably not get her PS+ unless there was some sort of deal like this.
or maybe allow at least two accounts to be linked together to share PS+
What you are proposing is basically gamesharing, which is a no-no. You can have your PS+ content on 2 PS3s and 2 Vitas, however they must all be under the same account...