Prospects for extending subway service to a suburban Maryland station at Wheaton and a Northern Virginia stop at Vienna have brightened as a result of a marked slowdown in construction costs, according to Metro officials.

Transit officials, who recently had warned of possible delays stemming from congressional limits on construction spending, now say the two subway branches appear likely to be completed on time.

The optimistic forecast was triggered by unexpectedly low bids for construction of the Wheaton station and adjoining 1 1/2-mile subway tunnels.

The lowest bid was a dramatic $32 million less than the transit authority and its consultants had predicted the work would cost. The bid by Ilbau Corp., an Austrian firm, was $51.5 million. Metro's estimate for the project was $83.9 million.

Although the transit agency has not yet determined whether to award the contract to Ilbau, Metro officials said the recent bids appeared to signal an easing of construction costs. Two other bids--by American firms--were less than $60 million, and all 10 bids fell below Metro's estimate.

"It really helps Wheaton," said John S. Egbert, Metro's assistant general manager for design and construction.

If costs remain as low as the bids indicate, officials said, the transit authority probably will have enough money to complete the Red Line extension from its present terminus at Silver Spring to the new Forest Glen and Wheaton stations by 1988, Metro's current goal.

In addition, Egbert said, the downturn in construction costs probably will allow the transit agency to achieve its aim of extending service in 1986 from the Ballston station in Arlington County, now a terminus of the Blue and Orange Lines, to the Vienna stop. The branch would include stations at East and West Falls Church and Dunn Loring.

Metro officials attributed the lessening of construction costs to lingering effects of the recent recession. Construction firms apparently slashed their normal profit margins in an attempt to win the Wheaton contract, transit officials said.

Despite the start of the economic recovery, the heavy construction industry has remained sluggish because of a shortage of public works projects undertaken by financially pressed state and local governments, limited investment in major ventures by manufacturing companies and a falloff in overseas construction work stemming partly from downturns in oil-producing nations.

When Congress was considering Metro's budget last month, transit officials had warned that a curtailment of funds would lead to a six-month delay in opening the Vienna extension and a year's postponement of the Wheaton spur.

Senate and House conferees later compromised on a $250 million allocation--$25 million less than the House recommended, but $20 million more than was proposed by the Senate and the Reagan administration.

Despite the brighter outlook for the Wheaton and Vienna branches, uncertainties surround other rail extensions.

Metro's plans to start service in December to three Alexandria stations and a stop near Huntington Avenue in Fairfax County are in doubt because of recent delays in testing new rail cars.

Shortages of funds and a court challenge have stalled plans for building Metro's Green Line through Anacostia and Northwest Washington to Prince George's County.

The Red Line extension from the Van Ness-UDC station on Connecticut Avenue NW to Shady Grove in Montgomery County is scheduled to open next year, but no date has been set.