Why does Nashville do this?
The Predators are going to be in cap hell with the flat cap, so after letting Mikael Granlund walk in free agency (and quite possibly Craig Smith too), Virtanen is a cheaper option that would slot very well into their top-6. He also adds more speed to a Nashville lineup that doesn't seem to be getting any quicker either.
Why does Vancouver do this?
If I remember correctly, the Canucks had Trenin very high on their draft list back in 2015. Nashville has been slow cooking him ever since, and he's on the cusp of becoming a very solid NHL'er who can play both centre and left wing. We wouldn't have to protect him in the expansion draft, and given that he'd be a much cheaper option than Virtanen, that saved money could be used to help the Canucks re-up Toffoli. Also, the MacAttack should be in the NHL full-time now.
I could go on all day about why I think it's best for Vancouver to send Virtanen packing, but that's a whole other discussion in itself... Let me know your thoughts below.