I would just be wary of "cutting my nose off to spite my face".
Personally I think I would book those flights from LA to Las Vegas now, and keep them as a "banker". Then hassle the living daylights out of your travel agent, and if you get no more joy out of them or Virgin for that matter, at least you know you will get to Las Vegas on time. Then try to get the money for those LA to Las Vegas flights out of someone.
If Virgin do re-route you on the way out just make sure that they don't cancel the return from Las Vegas on you, due to you not been on the outbound leg.
BA pulled that trick on another PPRuNe members better half recently due to an admin screw-up.