If you terminate shortly before vesting:
1. You face the prospect of an expensive and distracting wrongful termination suit.
2. You're sending a blindingly obvious signal to all other employees, and potential employees, that you can, will, and have terminated employees immediately before vesting. That may well prove ruinous if you find yourself unable to hire and/or retain talent.
Let the vest happen. Whether you separate from the employee before or after is your call, though if you've made that determination, do so.
PIPs are fugly as hell for all involved but exist for a reason. You should probably look into setting that up as a process.
Fix your recruitment, training, and monitoring processes.