Assuming when you say 'vest' you actually mean exercise. It's sounds like an investor doesn't believe you will execute without the extra incentive. I'm not a fan of tranches of any form, but sometimes you gotta do what you gotta do. Also, you didn't mention the strike price, so no way to know if this is a good deal or not. My advice is to ensure you have an experienced tech lawyer, and discuss it with them. They will have the experience to tell you what is normal and acceptable.
100% of investments I've been involved in (30+) over the past 6 years have never included these kind of terms. As Danny mentioned, using a "Tranche" approach to progress / investment is usually a clear indication that the investors don't trust the teams ability to execute.
Risk capital requires risk, and it seems like they're trying to mitigate it.
I would use their interest to approach other investors and get better terms. Obviously you're doing something interesting if they're interested, just take that momentum and find someone else who's going to be a real partner.