There is no vesting required for self-employed 401(k) (aka Solo K) plans, since you are the employer and the employee.
Vesting is a process in which assets that were completely owned by one party are eventually made the property of another party who has had use of the assets. In retirement plans, employer contributions typically have a vesting schedule, partially to give employees a reason to stick around for a few more years.
If you have a Self-Employed 401(k), there’s no vesting, because you are the employer. In other words, contributions made to your plan are yours immediately. Withdrawals may, of course, be subject to IRS rules and early withdrawal penalties.