[timr]

Yes. Never enter a business relationship with someone without setting those terms, in writing, before you begin. Likewise, don't dilly-dally around with starting as a "side project" with the real, secret intention of starting a business. If you suddenly think a project is a business, then from this day forward, you set the terms in writing, then proceed. Nothing from before the decision point matters, even if you've kinda-sorta worked on the "side-project" in various capacities for years. Just make a decision, divide the equity, set the vest, write it down, and move forward with confidence.

You don't necessarily need to have a full "legal document" at this point (unless actual money is involved -- then get a lawyer or service and do it correctly), but you do need to have the terms of the agreement clearly documented from the start. If you can't handle this conversation, you can't handle business. There are going to be far more stressful conversations in your future, and this stuff is table stakes.

The good news is that this is all so standard now that you shouldn't spend more than 30 seconds on vesting: "standard 4-year vest, 1 year cliff, starting from today." Anything more complicated or deviating from this is a strong sign that you're Doing It Wrong. In certain situations (i.e. you didn't follow this advice and worked for a while before formalizing the agreement) you might set the vest start date to something other than "today", but again, it's a warning sign. Try as hard as you can to be boring.