A few things:
1) This is a port that has had a lot of time spent on Vita optimization. Seriously. They announced this around six months ago and are still working on it. That's a lot of time for porting these days. It shows that they're really putting time into it. Thus, it should be a pretty good port, not like the Jak & Daxter HD collection was on the Vita.
2) They're porting a damned fine game, and there are people like me that have barely had the chance to play it who will happily jump right on top of this offer. All because you have both a PS3 and a Vita, thus, get the most out of PS+, doesn't mean everybody else does. Just because you have a home console and the means necessary to play the game does not mean everybody else does.
3) The free version was the no frills, vanilla Borderlands 2. Borderlands 2 on the Vita will apparently come with six packs of DLC included, which is really only fair considering the price of the GOTY version on the likes of the 360. In the long run, buying all of that DLC on the PS3 will equal, if not exceed the price of Borderlands 2 on the Vita, so really, was getting it for free on PS+ that great of a deal? Not to mention that your DLC you buy for that PS+ title is only good for as long as your PS+ membership is.
As well, like I said, this was announced over six months ago. If you're just now complaining because you don't like the idea of the Vita getting ports, you are pretty far behind.