all the big sites accept vectors, so deciding whether to go exclusive or not will basically depend whether you are in this to make as much money as possible, or whether you want the simplicity (and a few other benefits) of only uploading to IStock.
Obviously it is wise to try uploading your images to a number of sites for a few months, prior to locking them in with only one site, as you never know which site will make you the most money.
I currently upload vectors to five sites (istock, shutterstock, stockxpert, fotolia, 123RF) with istock making approx. 45-50% of my total income each month, so don't dismiss other sites without trying them as you can make good money.
If it's only a hobby/2nd income, you could go exclusive at istock for a number of good reasons:
1. Easier to upload to only one site.
2. Less likely to have images stolen.
3. Higher royalties.
4. Quicker approval times.
5. Images appear higher in search results.
6. Free business cards
Although you can also make quite good money from vectors on Dreamstime as well, be wary that there is a 6 month tie-in period for your images, that will effectively mean you can't go exclusive anywhere else.