Good, but not a requirement.
To start off in this game you are the person in the Animus unlike how it is Desmond in the normal games. Basically think of the VITA as the Animus and the game as a memory you are partaking in. I see what they were trying to go for, but it felt a little strange.
The game play is the same as the normal games, but just on a VITA so if you play on PS3 you should have no problem. I play on PC and had no problems adjusting to new controls. The story was ok. Nothing too interesting and it kind of takes about an act to get interesting. In the beginning you are going to be playing in the lady costume a lot which I find boring. After the first act or so things will get more interesting, however there is very little mention of the first civilization like in the main games. The mentions are there, but it does not play a big part. There is one part of the story where it gets a little more interesting . There are 3 costumes in the game: Assassin, Slave, Lady. Each costume could certain things. Assassin is the main costume. Lady will get you into parties, rich people areas, and you can charm men. The slave will let you walk around plantations and move quicker, but with less fighting abilities.
Overall this is a side story. It is good, but lacks that thing that makes the main games great. In terms of a side story it is a good play if you have the extra money or really love Assassin's Creed. You do not need to play it to fully understand the main games and it does little to help. Don't buy a VITA just for this game, but if you have one and can find the game for cheap or feel like playing it then go for it.