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.