We already own at Vistana, but tend to go to the presentation each year, just to see what they're selling. Anyway, you'll get there and sit down, a sales person will come and introduce themselves and invite you to get up and visit the breakfast buffet. (It's your basic continental breakfast)
Then they'll come sit down. Best bet is just be honest and tell them you're not interested. If it seems like a hard sell, (it isn't always) tell them you scraped every penny you had to do this disney trip and you can't possibly afford buying anything!
That said, if you want to buy, I would buy through TUG or even ebay. You'll get a much better price on someones resale. Vistana gets TOP dollar.