The Guardian is a useful paper, slightly more to it than most if you need a longer lasting fire in a hurry. The website is top too, with a breakdown of game sales by format (something the official chart doesn't bother showing).
So, we can see that FIFA 13 on Vita currently accounts for 3% of overall sales, which seems to have been a static figure since launch (and since the game sold a million in the first week, we can figure 30,000 sales as a base).
The reason I actually looked is to find Need for Speed on Vita is accounting for 1-in-20 sales which is a lot more than the PC version. That's not bad for the baby format on the block, hopefully enough to keep EA keen on the Vita. Check out the full Vita chart here.