1. They are part of a publicly traded company (Navarre,) so they could catch flack from investors/regulatory agencies for leaking information that could have an effect on stock price. This is part of why FUNimation tries to do stuff like the Roll-Out Riot (public announcement of properties,) and why even when they push stuff at a con, they often push press releases out on the same day - they could get in legal trouble otherwise.
2. FUNimation may be under contract with their Japanese partners to not announce the title until a certain date or until certain other conditions have been met. Violation of this by a party connected to FUNi could result in anything from monetary penalties to loss of the license.
3. FUNi has in the past and may very well still put VAs under riders that in the event of certain public transgressions jeopardizes their future employment with the company. In short, Vic could be kicked out of the FUNi casting pool entirely for leaking a license because of the contracts he signs when taking a project on. To put it another way, he probably signs a non-disclosure agreement when taking a dub on, especially when the title hasn't be announced officially.
In short, VAs have a lot of reasons to keep mum about projects, especially at events like anime cons, until they've been fully admitted to, especially when it comes to FUNimation.