I don't see the use for Montreal to do that trade now that we have Drouin on wing for the top 6. We need a center.

Honestly, it's less about that trade even. It's more or less just the Pietrangelo trade i'm looking for feedback. I'll revert the Perron/McCarron trade because i think that's what caused confusion

I don't think the Blues will be happy with the expensive Giroux in future years. IMO worry about getting centre next season when 7m Stastny's contract is gone. I can't see Vegas taking Lehtera.

You work out a deal for Vegas to take Lehtera. Hence a first round pick in this year's draft. Plus, what center will the Blues even be able to get? it won't be Tavares.

Everybody seems to think they a pick, even a good one, is going to convince Vegas to take a player they don't want. Well Vegas has cap as everyone else. I can't see them taking on a bunch of 3rd line players making 4m a year. How Blues fill that centre position? Don't know, they almost have to home grown. IMO Giroux is not the long term answer, production wise or cap wise, but that's me.

Well unfortunately it doesn't look like the Blues have anyone in the system that could be that number 1 guy either. So you either trade for one, or take chances in free agency. Giroux isn't my first choice, but the likelihood we get a guy like Kuznetsov isn't as likely right now

Rumors are still swirling that STL has a deal in place with Vegas. A pretty trusted source seems to think that the Lehtera thing is legit (https://twitter.com/jprutherford/status/875017309752090625) If the deal is Lehtera + 2nd + a prospect to VGK, I think the Blues should try to fill center by trading up to 4-8 OA with their 20 OA + 27 OA + roster player and grabbing a talented center in this year's draft. If that's not possible, there should be a few top six centers available via trade - RNH (EDM money problems), Henrique (Devils have a ton of young C's), Tyler Johnson (less likely now), Stepan, Duchene, etc.

(Knowing Armstrong's tendencies, he'll likely do nothing but re-sign the underperforming Stastny for too much and for too long.)

Lehtera is going to Vegas. I'm guessing like you said probably a pick and Blais.
I don't see Duchene happening because of Colorado's asking price. Last major trade we had with them was for Shattenkirk, and they weren't in our division at the time. RNH is Stastny more or less. Great for Yeo's system, but not the major offensive force we need. Stepan is interesting plus he's a righty. We need one up front for now. I don't like Johnson, even before the Drouin trade.
I think our best bet would be to try and get within that 4 to 8 range. Will it happen, unlikely again as you said, it is Armstrong. But I want Vilardi personally. I think he's a going to be a gem in this draft.