IIRC, the company doesn't have to pay you for unused vacation days even if you don't use them. It's pretty much at the discretion of the company.
Other states view vacation as just an agreement between you and your employer. You basically agree that there are so many days you don't have to show up to work and they'll still pay you. That's it.
In the past I worked remote for an Atlanta-based company, and their employee handbook stated that they will pay out vacation up to a maximum of 40 hours. Anything in excess of 40 hours you had in the bank, you forfeited. I lived in a St Louis suburb in Illinois at the time, and as an Illinois-based employee they paid me my full vacation bucket when I quit.
Similarly, a friend of mine worked remote for the same company, but lived just across the river in Missouri. When she left, she got her vacation capped. MO and GA are among the states that do not view vacation as earned compensation.
Since you need this check, you wouldn't get your vacation days.
Anyway, while I agree with the red flag concern, I don't necessarily agree that companies have unlimited vacation in order to cause you to take less. Esp in the startup world, esp at early stage, I don't expect to see this sort of intentional subterfuge - esp because of how it could backfire. I do think a minimum vacation policy makes more sense and allows, esp in places like CA where vacation is earned wage, and unused vacation is paid out at the end of the year, to manage the way vacation is counted.
I also think a company with mandatory vacation should consider allowing people to take at least some vacation in the form of a bonus. Maybe they have a spouse who doesn't have as flexible of a schedule or just really enjoy their work. I don't think this should be an option for founders, however, and I think it should accompany a discussion, because it doesn't work for most people.
You can write any old thing you like and get someone to sign it.
That doesn't necessarily mean it's enforceable.
Writing unenforceable clauses into fine print -- that you know are unenforceable -- is a legal bluff older than keeping records on clay tablets.
Usually the way this works is they give you severance, which comes at a price: you won't sue them for any reason whatsoever.
Doesn't help if you're fired, of course.