It's an ordinary parking garage, but when the four-level ramp at the Vienna Metro station opens Saturday, it literally will change the way thousands of Northern Virginians get to work.

Janet Gillman of Manassas, for instance. She gets up at 4:30 a.m. every weekday so that she can arrive at the Vienna station before the parking lot fills up, which usually is by 7:20 a.m.

"I'm usually half an hour early to work," said Gillman, a secretary at Riggs National Bank of Washington. "But it's a question of getting a parking spot or getting to work at all."

Gillman will be able to sleep later because when the new garage opens, the Vienna station will have enough parking so that people still will be able to find a space by 9:30 a.m., Fairfax County officials said.

The new garage at the station's northwest corner is the biggest improvement on the Orange Line since it was extended from Ballston to Vienna in 1986. As the Orange Line's western end-point at Interstate 66 and Nutley Street, Vienna serves one of the metropolitan area's fastest growing corridors from Fairfax to Fauquier County.

"It will take the anxiety out of whether or not people will get a parking space in the lot or risk parking illegally," said Fairfax Supervisor Kate Hanley (D), whose Providence district includes the station. "It will bring more certainty to people who want to ride Metro and it will take more cars off Interstate 66."

The $15.5 million garage was built over 13 months by Fairfax County and Metro with parking revenue bonds.

The garage is expected to attract more riders to the Vienna station. Vienna's rail ridership, which now hovers around 8,400 people daily, will increase by an estimated 1,300 people a day. Metro will take four additional train cars out of storage to handle the extra rush-hour loads.

With the additional 1,874 parking spaces in the garage, the Vienna station now will have 3,567 spaces, a net increase of about 1,300 spaces and more than any other Orange Line station in Virginia. About 565 spaces available at a private garage will be closed.

The garage's opening will mean that traffic leading to and from the station will be spread somewhat more evenly throughout the morning because everyone won't be trying to get there before 7:30 a.m.

Down I-66 at the Dunn Loring station off Gallows Road, the scarce parking situation also will ease slightly, because many commuters who now park at Dunn Loring will use Vienna. The Dunn Loring lot will fill up by 8:45 a.m., 30 minutes later, officials predict. No change is expected at the West Falls Church station, which is full by 8:15 a.m.

Metro and the local governments never have planned to have enough parking at stations to meet the demand; riders could fill 100,000 spaces, but only a fourth of that is built. The transit agency wants more people to use buses to get to its stations, and has offered reduced fares in some areas as incentives.

The additional parking at Vienna will be more expensive. The daily fee is being raised from $2 to $2.50 at the Vienna, Dunn Loring, West Falls Church and East Falls Church stations. Metro gets $1 out of that to operate the garage; the remaining $1.50 helps pay off the revenue bonds that built the Vienna lot and another garage at the Huntington Metro station scheduled to open next year.

Since giving up on Metro last year while the garage was being built, Edward Zager, of Oakton, said he has found that it is cheaper to drive to his job in Crystal City than pay the $4.70 round-trip Metro fare and $2 parking fee. Metro officials say they expect to lose some passengers because of the higher parking fee.

Other people complain that the new garage caters solely to rush-hour commuters and does little for people wanting to use Metro during off-peak hours.

Tom Gardner, for example, frequently goes from his Manassas engineering office to the District for meetings with clients during the day. He's been taking a taxicab to the station ever since "I drove around looking for a space at Vienna and then Dunn Loring and said, 'The hell with it.' "

The privately owned Hunter's Branch parking garage southeast of the station is a possibility for mid-day parking. The garage's 565 spaces that have been used by commuters while the new lot was being built will close next week. But county officials and the company that owns the garage said Hunter's Branch may reopen to commuters if the demand for spaces remains high at Vienna.

"If their lot fills up and they still don't have adequate parking, we always have the ability to sit down and talk about it again," said James Evans, executive vice president of The Evans Co., which owns the garage for its adjacent office development.

Chances are good that Fairfax officials will be back to Evans. The garage is expected to satisfy the rush-hour parking demand through 1995, and there always will be a need for more midday parking, officials said.

Right now, however, commuters are eagerly waiting for the end to four years of getting up early to find a legal parking space at Vienna.

"I don't know about you," said Luis A. Valenzuela, who leaves his Centreville home at 5:30 a.m. each day to park at Vienna, "but I'd rather stay in bed an extra 10 or 15 minutes, or even a half-hour."