The reality is, you should have fired this person months ago. You didn’t, for whatever reason, and that’s ultimately on you as the founder. That is your fault. Punishing someone who took a risk of joining your team early, even if you’re unhappy with their performance, by cutting their vest seems ridiculously petty and to me, is a sign of a bad manager.
Given the nature of startup options, it is likely the vest amount both won’t be substantial in the long run (if your company succeeds, you’ll raise further rounds that dilute early employee options) or, candidly, that none of this will matter at all because there will be nothing to vest.
This isn’t a co-founder. This is one of your first hires. You made a bad one. It happens. The time to fire pre-vesting was months ago. Taking away the vest mere weeks before it happens is something you would never do to an executive on your team if you were more established. Why would you do it for someone you made the poor decision of not just hiring, but keeping around long after it was clear their hiring was a mistake?
Pay it out. It’s a small thing in the grand scheme of things and hopefully can be a lesson to fire faster in the future when you bring on the wrong people.