Office vacancies run at a 6.1 percent rate in Montgomery County's more than 6.7 million square feet of office space, according to a survey by Spaulding & Slye Corp., which has an office in Rockville.

It also was noted that average rental rates of office space currently under construction rose $1.05 per square foot in a recent quarter. In Fairfax County, the vacancy factor was 4.8 percent -- with more than 50 percent of the available 286,000 feet being in the McLean-Tysons area. It was noted that 1.6 million square feet of space are under construction in Fairfax County and that 2 million more feet are planned.

Capital Publishing Co., which produces newsletters, has leased more than 40,000 square feet of office space at 1300 N. 17th St. in Rosslyn, where the company will move from a District location. Braedon Companies and Turner Real Estate arrranged the $6.8 million transaction.

Sky Courier Network, an international service specializing in the handling of fast deliveries, recently moved its headquarters and 170 employes from Tysons Corner to three new Commerce Park buildings in Reston. Sky Courier will have 44,000 square feet in structures developed by Centennial Contractors Inc.

Texas Instruments has leased 30,000 squrare feet in a new office building on Prosperity Avenue in Fairfax County from Dewy Associates. Lease arangements were negotiated by Town & Country Properties and the Carey Winston Co.

Lois Zambo, an assistant vice president with the Washington office of Julien J. Studley Inc., recently arranged for the Wilner & Scheiner law firm to take 5,000 square feet at 1200 New Hampshire Ave. NW.

Lreenhoot Inc., which recently opened its own leasing office in Falls Church, arranged for Cerberonics Inc. to extend its lease of 17,868 square feet in the same building at 5600 Columbia Pike, Falls Church, through May 1968.

Tenants in the nearly completed 14-story 8200 Greensboro Drive building at Tysons Conner will include Washington-Lee Savings and Loan Association, which will have its haedquarters and a branch office there. Other major tenants included Dynamic Systems and Resource Consultants, both of which do federal contracting research and development.

H.G. Smithy Co. has an exclusive leasing contract for an office building being developed by Wills & Van Metre Inc. at 9900 Fairfax Square in the City of Fairfax. The midrise building will have 50,000 square feet in a area with ponds, foot bridges, decks and gazebos. Completion is scheduled next summer.

Danac Associates Inc. arranged a five-year lease for 15,914 square feet in Ward II office building in the Shady Grove Road area of Montgomery County for NUS Corp.