Why are you asking about AMEX, if your point is to get them to VA? It might be possible to get to VA even if you can't get to AMEX (AFAIK, there's no way to transfer into AMEX MR from anywhere!).

But you should also know that transfer between airlines, even when possible, involves extreme loss of value. To the point where I would think it wouldn't even be worth considering. Why would 15,000 VA miles be more valueable then 100,000 AA miles (these, if not worse, are the kinds of transfer devaluations you typically get).

The one way around that is to try one of the "trading" services (where you use your AA miles to redeem an award for someone while they use their VA miles to redeem an award for you). One of those is Coupon Connection here at FT (and you have enough posts to qualify, I think), the other is GPX over at points.com.