Absolutely loving this game, played it about 3 hours last night on my Vita. Got an Elf up to 9 and Sorceress to 6. Game is obviously gorgeous as anyone would expect on the OLED, but the frame rate is a mess. It's completely obvious from the first level that the Vita hardware is not up to snuff at trying to keep a steady frame rate in this game, it's a disaster.
Anyone who had has any concerns about this problem should just straight up buy the PS3 version. It's bothering me, greatly, although I knew there would be framerate problems going in. I'm still glad I bought the Vita version, it's the way this game was meant to be played.