Assuming performance is not an issue (i.e I was a good worker) ... is there any recourse to being removed from a startup right before a significant equity vesting date?
Talk to them first... see if you can get your options or a prorated portion of them before contacting lawyers. But make sure that you notify them that you will pursue this matter through courts if you can't settle amicably.
If your dismissal is "obvious" (in the sense that it is done with a view towards denying your share) i.e., your performance to date has been good and you get the boot, as you say, "right before" a scheduled vesting date, then you probably have a case.
Enough to make an inquiry with an attorney, at least.