I understand that it's uncomfortable on a personal level (looking at it from both sides), and you might wanna consider the optics for company politics.
On a purely mechanics level I am somewhat confused: If the setup is such, that firing the employee can lead to cutting them out of the vesting at this stage, then that's because it has explicitly been built into the vesting mechanism. If that is not an option that you are actually willing to exercise, or if the employee is not okay with this being used as mechanism against them, why was this contract set up and signed by both parties?
If there's no good answer to that, revising the system in a way that everyone involved feels actually okay with seems like a good next step. Since I am not all that deep into vesting mechanisms, I am happy to learn why this is might be a dilemma situation that has no generally acceptable solution.
On a purely mechanics level I am somewhat confused: If the setup is such, that firing the employee can lead to cutting them out of the vesting at this stage, then that's because it has explicitly been built into the vesting mechanism. If that is not an option that you are actually willing to exercise, or if the employee is not okay with this being used as mechanism against them, why was this contract set up and signed by both parties?
If there's no good answer to that, revising the system in a way that everyone involved feels actually okay with seems like a good next step. Since I am not all that deep into vesting mechanisms, I am happy to learn why this is might be a dilemma situation that has no generally acceptable solution.