If I cast Sacrifice on my 5 might Vi and then opponent reacts with Smokescreen on the Vi, does the Vi still die to Sacrifice and I get the Draw 2 and Channel 1?

Yes, Vi dies and you get the Draw 2 and Channel 1.

Here's why: When you play Sacrifice, the additional cost (killing a friendly Mighty unit) is paid during Step 4 of the playing process — before the spell goes on the chain. By the time the opponent has priority to play Smokescreen as a Reaction, Vi is already in the trash and Sacrifice is already finalized on the chain. They can't save her or interrupt the cost (FAQ #9906, Rule 357.2).

Important distinction: The opponent's only window to stop this is before you play Sacrifice, during an Open State. If they Smokescreen Vi (reducing her to 1 Might) while you have priority and haven't cast Sacrifice yet, Vi is no longer Mighty (needs 5+), so you can't choose her as Sacrifice's cost. But once you announce Sacrifice and pay the cost, it's too late for them to react.