I'd take that rumor with a grain of salt for now. The Vikings to need make cap room first and some of the beat reports think it's unlikely.
While a 2nd and 5th is great value for Beckham, I'm not sure I'd trade Diggs for being a diva/complaining and then turn around and trade for Beckham who is a much much amped of version of that. Diggs just has a strong personality. Beckham is an actual diva WR and cry baby.
In this deep WR class and 5 picks in the first 3 rounds, I'd just draft a couple, but Speilman may be gun shy about taking a WR high after how Patterson and Treadwell turned out, which is why he probably called about Beckham (and I'm sure some other vets).