Basically, i look at this proposal as boiling down to: a 3rd round pick, in exchange for taking on that $4.5M Zaitsev cap hit for his 29-32 year old seasons. I don't think that's quite enough to consider it...especially since that's probably the range of years where the Canucks will be looking to start competing and dealing with the sort of cap crunch that's about to hit the Leafs as young core pieces come off their ELCs.
Stecher for Zaitsev as players right now, is probably more or less a wash. If Zaitsev rebounds to his rookie form, maybe it swings a bit that way, but if Stecher continues to improve incrementally, maybe it swings that way. But they're probably more or less comparable #4ish RHD. With a fairly significant salary gulf between them, for the time being at least.
The Gagner deal going to TOR obviously helps offset that salary gulf this year and next, but after that...it's just pure cap dump going to Vancouver for 4 years. Years that venture into the players' 30s where some decline is to be expected from a middling guy like Zaitsev.
So it'd have to be more than just a (probably late) 3rd round pick to justify it. And even then...i'm just not sure it's worth hamstringing the future cap flexibility from 2020-2024. Where if/when you end up paying guys coming off ELCs, you're probably spending more than a 3rd to dump those likely "worst years" of the Zaitsev deal to make salary space for others, as the Leafs would be doing right now.