Putting an HD seat on your Victory Vegas would very likely be an exercise in futility. Aftermarket seats are available. Have you checked the usual places like Facebook & eBay along with the typical Victory parts houses like Witchdoctors or Ultimate Seats? I see a fair number of Saddleman seats on eBay from time to time.
Also, since the seat pan on a Victory Vegas is unique, any reputable seat alteration company like Will Molina at Cycle Ops in Florida can re-do your stock seat into any configuration you like. Many have also used a local car upolstery shops with great results.