Vic Simas has been a NASA researcher, rebuilt player pianos, designed and put on the road an electric auto and dabbled in tournament bridge, but his first love is big-band swing. His 17-piece orchestra with vocalist will hold forth Friday at 7:30 p.m. on the grounds of the Montpelier Mansion (one-half mile west of the Baltimore-Washington Parkway on Rte. 197) in a free concert sponsored by the Friends of Montpelier and the South Laurel Recreation Council.

"I was a teen-ager when this stuff was coming out," Simas says, "and I always felt that Jimmie Lunceford and his arranger Sy Oliver started the whole thing, the real swing sound. Then, of course, Tommy Dorsey picked Oliver up. To me, the arrangements are the band and the arranger is the whole works." Three heavy suitcases of charts constitute the 3-year-old orchestra's book of more than 300 tunes. Trombonist Porter Lyon is chief arranger.

Simas--who played piano in Reno clubs in the late '30s at age 16 and was bassoonist with the Reno symphony while in college and saxist with a Navy band during WWII--formed his first big band in 1951, here in D.C. Several of the original members, including trombonist Jack Patermaster, are still with him, but the band's personnel also numbers some players in their late teens. Among the featured soloists are tenor saxist Rick Parrell, Patermaster and the leader on alto and clarinet. For the picnic-style gig, Jackie Schmitt will sing Count Basie's "Foggy Day" and the band will jump on Les Brown's "Leap Frog" and lightly swing "String of Pearls," the Glenn Miller hit of 1941. (For more information, call 552-1888.)