Is Symphony of the Night better than Castlevanias on DS? I'm thinking whether it's better to pick it up on Vita or just play the DS ones.
Don't purchase the PSOne classic of SotN on the Vita, that'd be a bad choice. Instead you should purchase Dracula X Chronicles(also available on the Vita.)
Dracula X Chronicles is a remake of Rondo of Blood, better than RoB imo with the changes that's made. It includes the remake, but also Symphony of the Night, Rondo of Blood, and a game that was playable if you didn't have the right system card to play RoB. Symphony of the Night included in Dracula X Chronicles includes redone voice acting, and while it no longer has the cheesy opening dialogue because of it, it's overall better. It also includes Maria playable for the first time unless you count the NitM Maria on Saturn who really played nothing like Maria. There are also extra weapons you can get, and you can now get the Axe Lord Armor in the regular game.
If you do choose to get a DS Castlevania, I'd recommend Dawn of Sorrow. Dracula X Chronicles is the better game as it is an overall better package including three games instead of one, all three being some of the best in the series. Dawn of Sorrow and Symphony of the Night are just as good as one another imo. I will say that I've played SotN a large number of times that my opinion on it is less than it would be if I didn't play it as much. It was one of those games that I replayed it over and over again the first time I played it, found out everything about the game and got kind of burnt out on it. I know everything about the game backwards and forwards and I think that kind of degrades my opinion of it in a way.