The problem for the legacy airlines is likely to be the HUGE gap in the main cabins. Anyone not always flying F/J will look stupid flying the legacy coach for around the same price. VA will have no problems filling their 12 premium seats with all paid seats. How many of the AA/UA/CO premium seats will be paid seats to justify their wide bodies?
Looks like whatever market share VA wants to take in these routes it is there for the taking especially in the beginning when the planes and interiors are new and the buzz will be there. How much effect it will have on the legacies remains to be seen.
The thing that VA makes dangerous for legacy airlines is that as a consumer branding company Virgin (and it will inherit that culture regardless of the level of investment from outside the US) will outsmart all the legacy airlines marketing combined (not that it is difficult to do with the type of people AA has got in its marketing).
Expect to see some very clever ads and gimmicks where the competitor's outdated cabins and service will be exposed brutally as your "father's oldsmobile". The target market for VA main cabins will not be the bottom feeding families that fly WN but the young and the chic students and professionals that will show brand loyalty (as opposed to FF loyalty) as they grow up.
All in all, it is great for the consumer!