Employee salary deferrals are immediately 100 percent vested — that is, the money that an employee has contributed to the plan cannot be forfeited. When an employee leaves employment, he or she is entitled to those deferrals, plus any investment gains (or minus losses) on his or her deferrals.
In safe harbor plans, all required employer contributions are always 100 percent vested.
In traditional plans, you can design your plan so that employer contributions become vested over time, according to a vesting schedule.