It depends on the cruise but we canceled one that we had booked on board shortly after the date was released and lost the deposit, obb credit, and 10% discount and after all of that, the VGT cabin we got was still cheaper and it was a higher category than the original veranda we had booked. This was on a CA sailing 2 years ago. We've done 4 VGTs, 3 out of California and they hadn't gone up much than first release date and the GT rates when released were def lower. Since their CA cruises out of LA didn't sell well, I was shocked to see how popular the ones out of San Diego were. I'm hoping it's like their first hawaii cruise and lot of people cancel - I was counting on a VGT for that and was surprised to see it almost sell out!
If there is a particular cruise you want to do, I would book it when it's first released and then watch the price and availability and if cruises before that date are getting discounted and then cancel and rebook as a GT before you lose your deposit.