Basically: Were you negligent?

Assuming you weren't, it's very strange to force someone out right before their equity vests.

Basically, your lawyer can understand the situation better than an internet message board can. Then, a phone call from your lawyer to your co-founder could help make your co-founder become a lot more reasonable.

I have worked with lawyers. Once litigation is threatened, everyone suddenly loses all the neutral stuff and becomes reasonable.

If you can afford it, find a good lawyer.

Whilst the investor might consider that taking sides in personal aspects of the dispute looks unprofessional and might not see your compensation as a priority, surely the only way they could actually be truly 'neutral' on the future of the business is if they've already written the investment off as a bad one. (Which in itself would be useful to know)

Otherwise I would expect them to be (i) interested in ensuring the underlying tech was maintainable in future, which if nothing else might result in more reasonable earnout possibilities for you and (ii) concerned that one of the founders apparently wishes to cut others out of the business to turn it into his personal cash cow, particularly if the other founder is the one with monetisation ideas.

It's not guaranteed that the investor is your friend (another possibility is that replacing you and taking the business in a different direction is something they quietly encouraged) but I wouldn't expect them to be 'neutral' on whether the business has a chance of generating them a return or not.

The typical tech lawyer has seen this exact scenario ~20 times. You probably have some leverage, and spending $1-2k to figure out how much and how best to apply it, given the lawyer's experience, makes tons of sense.

Exactly what you should do depends on a lot of details about exactly what your business setup looks like, what your personal situation is, and what the other parties to the situation think. Way more detail is needed than you'd provide to any of us internet randoms. A good lawyer who understands these kinds of contracts is somebody you can actually give all of the important details to and who can give you accurate advice for your situation. This is not a good time for some weird quirk of your contract or any related laws to surprise you. If your co-founder is being unreasonable, you need to know what your escalation options are, and you may need to communicate that you are prepared to execute on them promptly if they don't give you what you feel is a reasonable deal.

You don’t hire a developer just because they can say some jargon. They need to show proof of skill, either through code of theirs you can see, the interview process, whatever.

Similarly, business people don’t take shit like this seriously until lawyers are involved. They see no evidence of your ability to actually hold them to account and so believe they can just push you around.

Hiring a professional who knows how to actually hold the other side legally accountable, or make them hurt (tens of thousands of bucks in lawyer fees and months down the drain if the case goes to court) shows you mean business.

Otherwise it’s just talk.

A lawyer is also an expert and can give expert advice, both strategic (what should we do?) and tactical (how do we do it?) based on knowledge and experience. Exactly the same way you do in your technical domain. Would you want advice from a non-engineer on how to architect or build something? What database to use? No.

Like developers, lawyers cost money for a reason, and it’s because of the value they can bring.

Yeah, spend the money and a good lawyer. The 1000bucks I spend on a lawyer to send a you suing the wrong person letter to a company that’s now worth over trillion was the best money spend.