I have a job offer from my employer's competitor. I am not bound by any non-compete clauses. Colleagues often plan their departures to occur right after equity vesting dates and then pre-announce this with their managers a few weeks before. If I disclosed my intention today to join the competitor after the next equity vesting date, would my employer be able to terminate my contract immediately, denying me the vesting equity, without any other cause? Is this likely? The relevant space is tech companies in California.
California is at will. They can fire you instantly. However they tend not to do that- it makes people in the company upset. Its more normal that you give 2 weeks notice, and then they decide to just pay you to sit those 2 weeks if you're leaving to a competitor. But they're not forced to do that, just like you're not forced to give 2 weeks notice. It's just etiquette.
If you're really worried about the vesting date, I wouldn't hand in my notice until the shares are vested. That's the safest path. Try to push the start date at the new company to 2 weeks after vesting date, or give less than 2 weeks notice and accept you'll piss people off at the old job.
Generally speaking, it will not. except you have a reference case. Because generally speaking, if the company terminate your contract, they have to pay. Who would prefer to pay you when you decide to go??
If you really worry about it, then make sure your manager or HR don't know where you leave your current company. You could choose not mention where you go, or you could lie.
If you can not make sure your manager or HR know where you go before the last day you leave your current company, then wait for they know it. The less time they have, more stable your offer it is, and the less chance they would fire you.