1 hr 29 min is short, but a couple of things that make it viable as long as you buy it as a single ticket from VS or Delta.
Firstly, at JFK, they have immigration lines for transfer passengers which cut short the waiting. You bypass all the aliens (!) whose destination in the US is JFK.
Secondly, if VS fail to get you to the airport with enough time to transfer (and assuming you don't waste any time at JFK) then Delta will just put you on the next JFK-BOS flight. I would check there is one as a back-up i.e. don't get the last flight of the day.
Cheers
Martin