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The only time I actually found the slowdown irritating on the Vita was on Nightmare, and especially the final boss (though some other bosses are pretty bad as well). I didn't really play much after the patch was released. From the little I did play however, they did seem to have relieved the slowdown somewhat, but not enough for me to go back on my decision to not purchase the physical edition.
If you are not playing on Nightmare, I would say go for whichever platform you want to play on. But for Nightmare I really can't recommend playing on the Vita based on my pre-patch experience. Moreso, if it is the first time you go through Nightmare on the game.
I'm just going to add that I don't really care for portability, so that might be included in my opinion as well.