Vesting is your ownership of your account. You are always 100% vested in your own contributions and their earnings, including any rollovers you make to your account.

The vesting schedule for the matching contributions for employees assigned to work at Microsoft is one full year of service: A full year of service occurs when you work for twelve full months on assignment to Microsoft (measured from the first day of the first full month you are assigned to work at Microsoft) as a full-time temporary employee.