Every bonus I've gotten has vested. So its not that unusual. Generally vesting applies to larger bonuses and to bonuses tied to performance.
I'm guessing 1.5M is a larger bonus than the average tech employee gets each year but is pretty inline for a typical hedge fund.
In this case it seems obvious that it needs some kind of vesting terms to prevent people from taking the bonus and then just taking another offer anyways.
Hey, I believe you and have no reason to think you are lying. Every bonus I've had has been vested over a 2 year period with about 50% vesting upfront. Again this is probably due to my bonus being tied to performance which could change from year to year and the fact the its probably larger than the average bonus employees get.
Every company is different and every bonus is different. There is no one way a bonus is allocated.