Rarely, unless the option agreements or bylaws or other legal documents require it. And even here, there are often (but certainly not always) re-negotiated in acquisitions especially of earliest-stage companies.
And when they do “forward vest”, or accelerate … it’s usually a behind-the-scene negotiations between the CEO and the acquirer.
Part of the reason they’re buying you is the team. The entire reason, the main reason, or part of it … it varies. But part of it.
Vesting isn’t perfect. It’s a handcuff.
But it keeps people in their chairs.