What’s a fair exit package in this situation?

What is fair is to be open with your cofounder and investors.

Should I just keep the vested equity?

Should the other stockholders just dilute your holdings?

Is a cash buyout common or appropriate?

Probably less common that telling you to pound sand. Cash is the life blood of a company and giving you cash is very damaging to the startup.

To put it another way, you need to negotiate. To negotiate you need to care about the interests of the other people and in the case of a startup recognize that the vast majority of the work is in the future. And that there is no value in sunk costs. Good luck.