As the other answers have illustrated, it’s almost never worth it.
That doesn’t mean you shouldn’t do it. You should do the right thing IMHO. Just understand it likely will fall on deaf ears.
First, VCs are almost always going to back the CEO. So if you try to undermine the CEO, you will almost always lose. One simple explanation for this is the alternative is almost always worse. If you fire the CEO in most start-ups, there is no Plan B. Plan B is often to shut down the company.
Second, you instantly become Disloyal and an Outsider. This may not be fair. But start-ups are about charging the hill, together. If one of your VPs no longer believes in the vision, the CEO. Well, that’s a tough situation.
Third, you’re asking a board to confront itself. Think about this. It’s close to a No Win situation in a very small startup.
Now, I have been in a tough situation before. When I thought things were grim. What did I do?
Instead, I helped as much as I could. And then I made the best of what was there.
Very closely held companies are odd beasts.