Vesting:
Agency Automatic (1%) Contributions are subject to “vesting.” You become “vested” in (that is, entitled to keep) these contributions and any earnings
they accrue only after you have completed a time-in-service requirement — which is 3 years for most FERS employees and 2 years for FERS employees
in Congressional and certain noncareer positions.
All Federal civilian service counts toward vesting — not just service while you are a TSP participant.
The date your vesting period begins is determined by your TSP Service Computation Date (TSP-SCD), which your agency reports to the TSP record keeper. If you are a FERS participant, your Service Computation Date is shown along with the required vesting information on your quarterly and annual TSP participant statements. The date will never be earlier than January 1, 1984.
If you leave Government service before satisfying the vesting requirement, the Agency Automatic (1%) Contributions and their earnings will be forfeited to the
TSP. If you die before separating from service, you are automatically considered vested in all the money in your account.
Note: You are immediately vested in your own contributions and in any earnings they accrue. If you are receiving Matching Contributions, you are also immediately vested in those contributions and any earnings they accrue.
Source:
http://tsp.gov/forms/tspbk08.pdf
Go to "TSP.GOV", click on "Civilian" button for "forms and publications.", then "publications", then "booklets", then "
Summary of the Thrift Savings Plan (7/2009) - 29 pages - LPV 10/2008