It is undoubtedly precisely because VS' financial condition was known to DL that the purchase price was far less than SQ paid years earlier and barely - if at all - higher than the underlying value of the LHR slots that VS holds.
It is very likely that VS will see improvements but they are bearing the fruit of not being in alliances for a long time, of sticking to their 4 engine philosophy way beyond when it should have been abandoned, and failing to attract key corporate clients. Branson waited a long time to recognize that he needed people who really understand the airline industry and in bringing in Kreeger from AA, VS is willing to go after some of the best revenue in the business - and get back door access to archrival BA.
VS is the airline equivalent of a fixer-upper but it has everything that matters in real estate - location, location, location.
Let's give it a little time - but DL's "out" will always be that they can push to liquidate VS if necessary (or stand by to gain if VS cannot operate on their own any longer) and DL will still end up w/ half of VS' LHR slots.
But the chances are real high that VS will be turned around - but just like every US airline that has been closed to failing, it will take time to return to better days.