Felix is absolutely correct; VTG cannot establish a group after-the-fact. And there have been VERY few rates offered by them that your travel agent isn't able to get. In fact, many times we're able to get rates lower than VTG. Their advertising gimmick of 'up to 75% off' is just that; a gimmick and is basically not true. They're talking about selling cruises at up to 75% off the brochure rate. Nobody, and I repeat, nobody, not even including the cruise lines, sell cruises for the brochure rate. So since they're not offering prices that most agents can get, they're really not offering 'up to 75% off' anything.
As I said, their website is great for research, but once you find a cruise, then talk to your agent who will be able to get that same rate and often less.