There are a lot of assumption here...my thoughts are;
"If" the borders reopen and Europeans are allowed back into the US, you will be rerouted via LAX and then VA will put you on a codeshare with Delta to Vegas. I will be amazed if VA operate a direct London to Vegas route anytime soon.
If the borders remain closed, VA will cancel your flight and offer you either a voucher or refund. That's what they did with my Mum when she was due to come out in March.
I wouldn't worry at all about a 14 day quarantine. No one will enforce it here. I think technically that rule is in place here, yet we have people from California, Utah etc coming...no one is checking them. Hotels, restaurants etc are all desperate for business, so the powers that be will put a stop to any quarantine.
How the world looks today may look very different come November. Anything and everything might happen. Don't cancel your VA booking, and don't take a voucher if they offer you one. I know you are saying that you will want to fly next year, but it is questionable if VA will be around then, and you don't want to be stuck with a useless voucher.
If you haven't booked any hotels, there are some amazing deals to be had. If you have, double check the price against what you have now, you might save a ton.
Lastly, if you do make it out here, please support local owned restaurants off the strip. We as a city are hurting right now.
Hope this helps.