How's it been treating you?
I spent some time running around the tanker in MGS2 and it's Metal Gear alright, now with framerate fluctuations and terrible back-touch controls. No, it's not that bad, but noticeable regardless. Super smooth in corridors with nothing going on, slows down outside in the storm and hitches in weird spots, like when aiming down the room with the television and drink cabinet. I checked out MGS3 very briefly and didn't notice any problems with the framerate, just that it was lower overall, which might have also been preferable for 2.
Hasn't really affected gameplay, but the back-touching sure has. In order to tiptoe while aiming down the sights you have to do and hold a multi-touch gesture where you drag two fingers in the opposite directions while holding down R and Square and keep your thumb on the analog stick to actually aim. Leaning to either side is almost as big of a hassle, but you only need one finger on the back touchpad. Two mechanics I won't use anymore, way too unreliable.
I'm not sure if it'll be a problem, but I find it odd that you can't equip or unequip your current weapon or item while your back is against a wall. You can peek around corners with the touchscreen - why would you want to do that - but you can't tap the item icons.
Disregard all that, I think it plays well and looks great on the Vita screen. I was sceptical about aiming with the fiddly analog sticks but it's been a cinch.