If the Canucks don't want / need a first out of the deal, I can certainly see the Leafs parting with a top prospect.
As for the "LHD" -- Canucks have Hughes, OEL, and Kyle Burroughs.... sure, Hughes is great, and OEL is still pretty solid, but Travis Dermott is a fiarly substantial upgrade on Kyle Burroughs. He's also only 24 years old and shown the ability to play the right side.
Ultimately, I think Kerfoot makes more sense as the "salary" than Ritchie does. He's a better player, while Miller does everything that Kerfoot does, and if doing that, perhaps the Leafs can keep Dermott instead of having to do the Dermott-Schenn swap.
Maybe you do something along the lines of:
To Vancouver: Kerfoot, Robertson or Amirov, 2022 2nd Round Pick, 2023 3rd Round Pick
To Toronto: Miller @ 50% Retained, 2022 3rd Round Pick
If the Cancuks had a 2nd this year, then yes, forsure, give up Toronto's 1st and get Vancouver's 2nd back, and don't really have to value it on the same level as a Robertson/Amirov, but Toronto is already woefully short on picks, and Vancouvers 2nd best pick in the draft is Winnipeg's 3rd rounder.