YC includes a vesting clause as a standard part of their incorporation documents, in part for just this situation.

I've heard that a corporation was formed BEFORE the YC application. If shares were issued in that corporation without a vesting clause, then that's that.

If shares were not issued and the YC application is the only evidence of a 50/50 split, then why wouldn't one assume that vesting was part of that agreement? The question on the application was "describe the planned equity ownership breakdown...". Someone could reasonably argue that they agreed to a 50/50 split given the assumption of a vesting schedule, and would not have agreed to it otherwise. Which is to say, if the YC application is the proof of ownership, then how can you ignore everything that is also implicit in that application?