The whole *A lounge access is probably one of the more confusing set of rules in travel.

My understanding based on various posts and the UPGRD podcast is as follows:

As a *G you have access to all *A lounges with any *A boarding pass.

The exception is if you are *G through United, US Airways or Continental, you can not access RCC, US Air Club, or President's Club in the USA on Domestic ticket. You can access through with international boarding pass.

Since you are a US Air Club member, you have USA access to the three clubs through reciprocal agreement and are able to cover the gap of the exception above.

So you are right since Virgin isn't a *A partner you may not be allowed in LH Lounge. If you are connecting through, you may try that. So RCC would be your sure bet in this case.

I rarely fly into IAD, but I will certainly try and report back next time I do.