Vikings suffered a bunch of injuries on D and given the way the season has gone for them, they don't look good to make the playoffs being 1-5 now. Trade looked good on paper as they were expected to be a playoff team, but hasn't worked out for them this season.
They are in tough with the salary cap, so at least they got some picks back.
If they are in position to draft a QB, they have to do it. And whether they take the hit and move Cousins (say to SF), they may simply be better off doing so. They restructured his deal last off-season to make cap space this season and that obviously has blown up on them. That's how the salary cap works. NFL gives you the opportunity to delay the payment, but you always end up paying for it in the end.
Like NE is eating dead cap for Brady and NO will eat dead cap for Brees should he retire after the end of the season.