Jarie's answer is a good summary of things to consider. I think what you are referring to (given your request for "vesting language"), however, is a Founder Stock Purchase Agreement which applies to a corporation. The alternative is to have both a Stock Purchase Agreement and a Founder's Agreement. The former is slightly more common. If you decide to form an LLC with partners, you should all of the follow as well but you put it in the company's operating agreement.
Here are several factors that you should always consider in a startup founder's stock purchase agreement:
- Ownership - What are you purchasing your ownership with and how much
- Restrictions on ownership transfers - You only want original founders (with some exceptions) to own the founders stock
- Vesting - If a founder leaves early, how much ownership do they not get
- Investment Representations - The founder understands the nature of the business such that he is knowledgable to invest his own money (only for corporations).
- Employment Implications - The agreement is no guarantee of employment at the company
Some good founder stock purchase agreement templates for a DE corporation I have found thus far. Please go over these with a lawyer.
- UpCounsel - legal startup - Founder Stock Purchase Agreement 1
- Orrick - established SV law firm - Founder Stock Purchase Agreement 2
I am not affiliates with either entity. I have heard of Orrick but have never heard of UpCounsel.