US is in the section of T1 right next to T2 and there is now an airside connector, or there will be soon: can't remember as I have avoided using US as much as possible. In other words, AA is there for the long haul unless VX goes through a dramatic expansion and T2 usage needs to be re-evaluated. Delta is building a Sky Club in the T1 part that is accessible, so an option will be to get the Amex plat and use the Delta lounge, as well as the centurion at LAS.
As I use VX more, I would not advise using the Admiral's Club. By far the biggest benefit of the membership is the help and support they provide with AA flights. You will not get that as a VX passenger of course. Other than that, there is basically no sense at all in buying a membership for such a space, when you have a terminal like T2 at SF.
For me, one of the things I actually am looking forward to more with VX is taking the occasional trip to the Clubhouse. A 15 minute walk is no bother at all for a pleasant late lunch or early dinner and, with summer hours being what they will be, both will be an option most days. I spent many hours there back in the day, I remember flying out the day after the liquids affair all those years ago and LHR and most US airports were in chaos, the VS flight actually left, but a good five hours late, we had it lucky spending all that time in the clubhouse!