Is that if they total the amount of $600 or more in one year or per event?
That sounds right, but in my experience it's a good idea to get the W9 before you pay them anything. Vendors have a tendency not to get you that information when you need it, especially if you only use them periodically. I have found the best way to be sure you get the W9 is to have them submit it before you send the first payment.
The best time to obtain the W-9 is prior to the first payment. The second best time is prior to the payment that puts you over $600 for the year.