Your employment agreements set up the economic incentives for working at your company. If the agreement is 1 year cliff to vest and the employee took advantage of that why would you get mad? You got a years worth of work out of that person and in exchange that person got his/her stock. Sounds like a fair deal to me.
On the other hand, firing an engineer right before he/she vests is unethical because you are getting almost a years worth of work without having to pay out the stock. Also it is a pretty good way to get sued by said employee.