As a general matter, no--a person may retire (or resign, or quit) whenever they want, including while under investigation. Vesting in any pension or stock plans would be governed by those plans' normal terms--i.e. you may or may not have sufficient time in service to vest depending on when you retire, but that is a separate issue from your right to retire. However, you mention a union: if you are talking about early retirement (presumably with some benefits) under the terms of a union contract, you'd need to review the specific terms of the union or collective bargaining agreement to see what it says about this situation. Whatever the contract says, goes.