This would be highly unlikely. VS has much better name recognition on BOS-LHR than DL as they've been flying it since 1991 vs. DL who's only been on it for a few years. VS is so committed to BOS that they spent millions on building a VS Clubhouse @ BOS, one of only a handful of stations to have one. I don't think VS will be retreating from BOS.
Its not retreating, VS would still be in the market, but with DL operating the flight.
And the Clubhouse stuff was more than likely all planned out before DL took on the 49% stake. It would make sense for VS to leave and shut down the expensive club and allow DL, who has clubs and more mass to spread the costs out, to fly the route. On top of that, as said in this thread, the 787 is a lot of airplane for the route.
But it is plane size and financials. The 789 is quite a bit larger than the 763 to begin with. It also is much more expensive, in part because its new and provides better fuel performance and in part because you're buying a plane that can fly much longer routes than the 763. It's a lot of plane for a BOS-LHR route, regardless of seating configuration.
and this is the reason DL is having a hard time going 787 for the 767 replacement. They are going to pay a big price for a jet that can fly 12-15 hour flights when what they needs is a jet that can fly 6-12 hour flights. The A330 would be a better replacement BUT the capacity is much larger than a 763. That is why most think DL will go 330 on the 767 routes that can take the capacity but also end up with 787s and even 338s for the 767 routes that can't handle the capacity.
Also, rumor mill has it DL is going to invest in Terminal A and the SkyClubs.