Virgin could use bmi as a feeder airline for its long-haul network
Realistically they could feed from ABZ / EDI / MAN / BHD / DUB but that's the only routes I think that would generate enough feed to make it worthwhile. Perhaps they could make the mid haul routes feed the US routes but given they tend to arrive late morning they'd need to rejig the schedules a lot.
Merging long haul VS with short haul BD is a mantra that's been around for years but given that BA and BE * took about a decade to merge properly, it's not that easy to pull off. Remembering that VS just settled with the unhappy pilots, do they really want to open negotiations to merge the seniority lists?
* Not flybe, think about it