an on-going complaint about v-fest is how late they wait before publishing the line-up. i don't remember, but i'm pretty sure it wasn't out by may (maybe a headliner or two, but certainly not the whole thing).
if you're hoping to attract out-of-towners, you need to get on their schedules ASAP so they can plan (and not commit to other destination festivals). i suspect VFest lost a number of potential attendees to APW (same weekend) which released its line-up long before.
i assume delays in the past have been caused by waiting for one headliner to sign on. festivals generally want to advertise all big names during the first line-up announcement, in order to maximize the impact of the initial media blitz.