The Virginia House, in a surprisingly easy vote, today approved a mandatory seat belt bill that if approved by the Senate would make the state only the fourth in the nation to require their use.

"Thank you for buckling up!" read a sign that Del. J. Samuel Glasscock (D-Suffolk) thrust into the air seconds after the 57-to-39 vote.

Glasscock sponsored the bill that would require persons in the front seat of vehicles to use seat belts, with a few exceptions. The state already requires children 4 and under to ride in safety seats.

The measure now faces a routine final vote in the House, probably Saturday, before being passed on to the Senate, where the chairman of the transportation committee, Charles L. Waddell (D-Loudoun), said it has a good chance of passing.

Today's vote came after the bill survived moves to dilute or kill it in committee earlier this week. Those attempts failed when they met the strong support of a broad range of business, medical and consumer groups who said the bill could save the lives of as many as 200 Virginians a year.

Virginia would follow New York, New Jersey and Illinois as the first states to pass some type of mandatory seat belt law. The Maryland legislature killed a seat belt law this week. Some version of seat belt safety is pending in 31 other states, according to published surveys.

Opponents contend the Virginia bill is an infringement on the right of the individual to decide to use seat belts. They also object to a federal regulation that threatens to require airbags in automobiles if states with two-thirds the country's population do not pass mandatory seat belt laws before 1990.

But only two opponents raised questions on the floor during time for debate on the bill today. In a bow to some critics, the bill specifically says that failure to wear a seat belt could not be used as evidence to prove negligence in any court.

If passed, the law, which would go into effect on Jan. 1, 1986, would set a $25 fine.

"What we're trying to do is encourage people to use seat belts," said Glasscock, "we're not trying to create an unnecessary penalty."

The bill exempts mail carriers, law enforcement officers in some situations, persons with written medical excuses and some newspaper delivery persons. It would be the responsibility of the driver to make sure that front-seat passengers from 5 to 16 years old wear seat belts.

Auto dealers and manufacturers, reluctant to spend millions of dollars on airbags, have been among groups supporting the mandatory seat belt laws.

According to Glasscock, in 1982 and 1983 there were 1,387 traffic fatalities in Virginia and only 35 of those, or less than 3 percent, were persons wearing seat belts. Only about 20 percent of Virginians now buckle up voluntarily, he said.

A National Highway Traffic Safety Administration study showed that occupants not using seat belts are 3 1/2 times more likely to be killed and three times more likely to be injured in auto wrecks than are seat belt users.

Del. Mary A. Marshall (D-Arlington), speaking in favor of the bill on the floor today, told of two nephews who had been in separate auto wrecks. One, wearing a seat belt, emerged with a scratch on his face. The other is now a paraplegic, she said. "I urge you to support it," Marshall said at the end of her brief remarks.