Definitely get MGS HD. Although if you played all the games in their full, Substance/Subsistence formats, you may not want to pay $50 for a graphical upgrade.
Although Peace Walker with awesome controls surely does help, plus online play.
funny, i was just playing catherine a few hours ago. also on PS3. and you're lucky, i want rayman origins but i'm waiting until i can afford another $40 and that's the next vita game i'm getting. rayman will be the last vita game i buy until disgaea is released.
i was also just looking at the MGS collection. i've played every MGS game aside from the twin snakes on gamecube. 3 and 4 are my absolute favorites, and owning them on the vita is going to rock my fucking socks.