I would say that a lot of the answers here are essentially dishonest. "Pretending to not recognise a former boss that you obviously do recognise" could be the plot for a Friends or Seinfeld episode - there are so many obvious ways for that to go south.

My vote goes for chilly politeness. You do remember him. And you don't remember him fondly. He does have a reputation, and it isn't a good one. He burned his bridges with you, no reason to pretend otherwise, and that's going to come back and bite him in ways he doesn't want to.

As a lowly engineer I've burned suppliers over the way the company rep treated me. There was this one guy who just loved to do that thing where he wears a chunky ring on his pinky and deliberately presses it painfully into your hand as a power play when shaking hands. Well all that got him was closer scrutiny from me and I stopped giving his company the benefit of the doubt - doesn't sound like much, but lots of long term suppliers don't realise they survive on their reputation for not fucking up supply more than whether or not they actually fuck up, and when that reputation disappears they get the full focus of supply chain. Companies who just tick along tend to just tick along, don't start none won't be none. Companies who have a dick for a sales rep get attention.

If OP is going to be the customer in this scenario, and Evil Boss is going to be on the vendor team, I personally think it's perfectly professional to say something like "this all sounds great, but I've got concerns about this vendor's ability to meet their end of the contract given what we know about Evil Boss's manipulative and dishonest reputation and he seems to be a big part of their team."