i just got a vita 2 days ago it is a great little system if u can get one for the right price and if u get ps+ a new game or 2 a month is real nice for $70 a year.
PS+ is only $50 a year if you can afford the one year membership in one shot. Otherwise, yeah, the $72 isn't a bad deal either for just picking up a three month card four times a year.
Anyways, OP, I'd say go for it. You already have a 3DS that can do everything you may want a 3DS to do via tools such as Gateway, no matter how questionable they may be at times. As was already mentioned, the Vita is great for visual novels (VLR, Danganronpa, etc.), and it's amazing for RPGs, especially considering your access to PSP and PS1 games as well via PSN. It has some solid action/adventure games (Gravity Rush and Uncharted, for example), and what is easily the best platformers of the past couple of years (Rayman Origins/Legends) run fantastically on the system. Although I've already mentioned a couple, the system has some great exclusives already, and although it has a lot of ports, nearly all of them are top notch and a lot of fun on the Vita.
All in all, I'd call the Vita a better value than the 3DS based on what you get and what you can do. Although it isn't the machine for everybody, it has quite a number of things that can appeal to the majority easily if they stop and take a look. I'd say that there's a good chance that you would be rather happy with a Vita in hand.