I think I would let Vegas go where he is happiest too. The saying that the best way you can show how much you care for your dog is to know when to let them go, doesn't just apply to when they are on their way to Rainbow Bridge
.
As you said, he is thriving being an only dog and you know where he is and that he is well looked after.
Rather than transfer ownership completely, you could always say that he is 'on permanent loan' and that you will be responsible for vets bills etc. That way you can still retain some sort of control, so that if their circumstances change you know he will be brought back to you, rather than passed on out of your reach.
Perhaps Vegas is trying to tell you something- that he is moving sideways because there is another pooch out there who needs you?
These sort of things rarely happen without good reason.