Who cares if it was malicious? So it was a dick move. That's just more evidence that Donald Trump is a dick. But I'm not concerned with how much of an asshole Trump is, I'm concerned about democracy. And that leads me to being much more concerned with the fact and manner in which he was fired, not the timing of it.
You're right, if he was fired without an appropriate process or without cause (I haven't seen the IG report but it seems likely), that's a problem. But it's a problem independent of the maliciousness of the timing wrt pension vesting.
I work for a company that awards substantial stock options on a vesting schedule. If I get fired before they vest, I either get nothing or a pre-determined percentage of the original award. Sure, if I get fired the day before they vest, I'm going to look very carefully at their reasons for firing me (it might be at will employment, but they still need a reason for the timing). But assuming they fired me for a decent reason, I have no right to demand the unvested portion of my stock award.
The crime here is not the pension issue - if indeed McCabe had done something fireable, he has no reason to demand the pension that hadn't vested yet. If he hadn't (as seems likely), the real problem is his termination and how it came about, not his pension.