The site is at living dice, and here's last year's list. I would guess at least 75% (maybe more?) of the vendors on it will be back this year. Some of the small fry maybe not, but all the larger vendors usually are regulars at Gen Con.
One thing you can do is go by your friendly neighborhood gaming store. Check out the games they have, and talk to the people there about your gaming interests, and have them suggest games that might fit. Make those a priority to visit in the hall. Also, take note of the prices of games at the store. If you try the game at Gen Con, and find you like it, consider buying it locally unless the vendor is running a good deal (also avoids having to schlep it back). IMO, it's always best to support the local game store, unless it's a very small vendor, where they could use the extra revenue from the Con sale.