This is simple-the crew have to give 7 days notice, so you can call up to cancel after having found out whether your VS flight is operating anyway, and before checking-in.

The ticket will most certainly be flexible if you have not checked in, as you will not be on the flight manifest. However, once checked in, you need to speak to BA to get yourself offloaded from the manifest, at which point I would expect refunds to be harder as a reconciliation with the final manifest would have to have been completed.

The answer will be with someone who understands the booking system in detail, which I imagine is few of us.

You could of course just call BA and ask...