The problem is optics. Obviously from OPs perspective the vesting isn't part of the firing calculation. However from the perspective of another employee if they see someone get fired right before vesting then it looks like a bait and switch "we'll let you work up to vesting and then fire you to save money in the long run".

There's no obvious answer to when is it fine to fire someone before vesting. The day before is obviously not OK. 1 day after they started working obviously isn't an issue.