Vehicle license stickers for 1981 go on sale Monday in Fairfax County and the easiest way to purchase them is by mail, according to a reminder issued by the county.

Residents whose vehicles were registered in 1980 should receive license applications in the mail. Those whose applications bear the notation, "P.P. Taxes Paid," indicating that personal property taxes on the vehicle due Dec. 5, 1980, were paid, may purchase stickers by mail. Two weeks should be allowed for processing and mailing of stickers.

A taxpayers who has changed automobiles since last year, but who paid personal property tax on the old car, may use the form received in the mail if it notes taxes were paid, but should use a ball point pen to change the description of the vehicle on the form and enter the correct make, model, serial number and other data.

County license fees for 1981 are $20 for vehicles weighing less than 4,000 pounds; $25 for vehicles weighing more than 4,000 pounds, and $8 for motorcycles. New county vehicle license stickers must be displayed by April 16.

People who receive forms that bear the notation, "P.P. Taxes Not Paid," must pay overdue taxes at the Office of Finance in the Massey Building or at any of the county's six branch assessment offices.

Stickers may be purchased from March 16 through April 15 at the following places:

Massey Building, 4100 Chain Bridge Rd., Fairfax, weekdays, 8 a.m. to 4:30 p.m.

Brtanch assessment offices at 6210 North Kings Highway, Groveton; 1437 Balls Hill Rd., McLean; 6121 Franconia Rd., Franconia; 6507 Columbia Pike, Annandale; 6140 Rolling Rd., Springfield, or 11415 Isaac Newton Square South, Reston Building, on Mondays, Tuesday, Wednesdays and Fridays, 8 a.m. to 4:30 p.m. and Thursdays, 10 a.m. to 6:30 p.m.

Stickers may also be purchased at the North Post Building, 1903, Fort Belvoir and at the following county fire stations weekdays, 10 a.m. to 4:30 p.m. and Saturdays, 9 a.m. to 2 p.m.: Annandale, 8914 Little River Turnpike; Baileys Crossroads, 3601 Madison La.; Burke, 9501 Burke Lake Rd.; Chantilly, 4004 Walney Rd.; Fox Mill, 2610 Reston Ave.; Franconia, 6300 Beulah St.; Springfield, 7011 Backlick Rd.; Great Falls, 9916 Georgetown Pike; Jefferson, 3101 Hodge Pl.; McLean, 1440 Old Chain Bridge Rd.; Merrifield, 8739 Lee Hwy; Mount Vernon, 2701 Sherwood Hall La.; Tysons Corner, 1560 Spring Hill Rd. and Woodlawn, 8701 Lukens La.