I've never liked the Baldur's Gate series, either. When it comes down to it, I simply do not like WRPGs. I do not think that more traditional RP elements translate well into video games, and while I normally have no problem with typical Western fantasy cliches and tropes, I tend to think they don't work as well in video games. It's just not a style of video game I've ever enjoyed, and eve the WRPGs I have liked (Diablo, Dragon Age, ME2), it's sure as hell not the plot I enjoy about it, and I certainly don't like them as much as I have most FF games.
And as far as the "open world" vs. "linear" argument goes, open-world games have never been able to hold my interest very long. There has to be at least some linearity involved, stuff to keep you going in the plot, to keep me playing.
FFX-2...don't get me started. Don't get me wrong, I liked the game a lot the first time I played it. Subsequent playthroughs, though, were like "...what the hell is this?" and I'm now shocked that I ever liked it. The fact that you can get through the game without ever really interacting with the plot outside of cutscenes is the biggest offender. There is nothing romantic about the Lenne/Shuyin subplot - it's more of a "I'm a creepy stalker boyfriend" thing. The dresspheres were a neat way of bringing back the class system but they were poorly balanced and there was little point to ever using most of them. Ush can comment more on how broken the actual game mechanics were. And let's not forget the fact that it stomped all over the ending of FFX, which was awesome if not a typical "everyone lives happily ever after" ending (guess what - not all endings HAVE to be perfectly happy and cheery).
Anyway.
About Vanille - yes, she is 19. As to why Lightning and Snow always call her a kid...I'm going with the fact that she seems very young and immature. But she is indeed 19 years old. I would strongly suggest not looking her up in the Final Fantasy wiki, though, as there's some extremely huge spoilers on her page.