Apartment hunting in Fairfax County is tougher today than at any time in the last eight years, a government survey shows. Only 2.6 of every 100 apartments in the sprawling suburban county had openings for renters in 1981, the report says.

A nearly stagnant house sales market has kept many would-be homeowners in rented apartments, helping to push the rental vacancy rates to new lows, said Jeff Bates of the county research and statistics office.

In densely populated Alexandria and Arlington, where apartment rental markets traditionally have been the tightest in the Washington metropolitan area, county officials said vacancy rates have increased slightly in the past year.

The latest surveys for Alexandria show vacancy levels of 1.97 percent--still lower than Fairfax's rates but up from Alexandria's decade-low of .7 percent in 1980. The most recent figures for Arlington showed a 1.37 percent vacancy rate in 1980.

The condominium boom that has been gobbling rental units in Alexandria and Arlington for the past decade has now hit hard times and is spitting rental apartments back into the market, according to John T. O'Neill, vice president of the Apartment and Office Building Association of Metropolitan Washington, a group that monitors rental vacancies.

"People thought that there would be a tighter market in the rental stream because of conversions to condominiums," said O'Neill. "But 30 to 35 percent of all condominium units in Northern Virginia are being renter-occupied."

"People who bought condominiums two years ago and have now left the area aren't able to sell," said O'Neill. "Now they are renting them out." Many condominiums also have been returned to the rental market by owners who purchased the units as investments, he said.

But after becoming condominiums, the units are being rented for significantly more money than the original apartments, surveys show. And county officials say the highest apartment vacancy rates are in those high-rent complexes.

Preliminary reports from the Fairfax County 1982 Rental Housing Survey indicate that the vacancy rate for higher-priced apartments in high-rise buildings has risen from about 1.5 percent in 1981 to about 3.5 percent in 1982.

Arlington and Alexandria officials say the luxury, high-priced units in their areas also have the highest vacancy rates.

People looking for low- and medium-priced apartments, however, are finding a scarce supply throughout Northern Virginia, according to county officials. Most low- and moderately-priced rental complexes are operating at full capacity, many with long waiting lists, officials said.

Renters with children are having even more trouble finding apartments, said Mark Looney, the landlord-tenant administrator for Alexandria.

"About half of the rental complexes in Alexandria either prohibit children or put limitations on the number of children they will accept in one unit," Looney said.

Fairfax County officials blame their overall high apartment occupancy rates on condominium conversions, lack of construction of new rental units and the increased number of renters forced to remain in apartments because of the stagnant housing market.

Arlington and Alexandria officials attribute their declining vacancy rates to another phenomenon.

"The decline in the federal job sector is discouraging more people from moving here," Looney said. "And a certain number of people who have been living in apartments have been riffed and have left the area."