Is there a rough day when the vendor list/map would come out so I can compare to my games at home to know where I want to visit? Also is there ever a list of exclusives or promo items?
On the interactive map, there are links to each vendor's home page. You can go there & see if they have any goodies for Gen Con.
Looks like the map came out in May the last two years. I imagine there could be some variation as Con dates shift and the like, which presumably is why Mike (wisely) didn't commit above to specific dates for this year.
When we get closer BGG has a list of what vendors are bringing what and if they have promos and such. Its usually a pretty exhaustive list by time you get about 3-4 weeks out.