Oh, the players responded positively, even though it made their life more difficult because Vecna showed up while they were in the middle of the battle. (They didn't necessarily know that's what the timer was for but I don't think it was hard to guess.)I think I was more curious about the effect of triggering an event in realtime than an actual play report since most of that 5e(?) crunch went right over my head. I've done 5-10 sec countdowns. Often out loud. But never had a 'this needs to happen before the end of the game session' trigger before.
All of us knew we were there specifically for a test-fight with Vecna. I view the metagame timer as a way of respecting the players' and my own time--to put it another way, a way of avoiding a pathological outcome where the main event is an afterthought. The lack of negative outcome is how I know it worked.
I wouldn't use it in a scenario where there was an in-game reason for Vecna to arrive or not arrive at a particular time, but I've used such things for events that are essentially random from the player's perspective: e.g. someone has been kidnapped, and they might get executed at any time, but since any time is as good as another, the kidnappers might as well kill the hostage at whatever moment happens to coincide with the players and DM running out of time instead of at a time determined by rolling dice. They are both quasi-random methods of choosing a time.
I've also used it for things like poison expiration: "the drow sleep poison you just looted off the fallen drow warriors will last until, um, the end of next month in real-world time." Same logic: when an event must happen but the timing is arbitrary, might as well pick a time that's convenient for everyone and that empowers the players to make good decisions.