Lawyers for a McLean-based research firm, accused of illegally obtaining a $2.6 million Army computer contract, have asked a federal judge to overturn a series of criminal charges against the company and its employes, complaining of abuses by government investigators.

Brian P. Gettings, a lawyer for General Research Corp., a Flow General Inc. subsidiary, asserted at a U.S. District Court hearing in Alexandria last week that the government illegally used military personnel to conduct a civilian investigation and that prosecutors showed improper vindictiveness.

Judge Oren R. Lewis rejected the lawyers' complaints in part, but said that he would rule later on several issues. The trial is scheduled to start May 24. It has not yet been announced whether Lewis or another judge will preside.

General Research Corp. and five employes, including a senior vice president, were charged with fraudulently conspiring to secure the contract for a computerized Army personnel system, awarded to the company in 1980. The indictment also alleges violations of bribery and conflict-of-interest laws.

Prosecutors contend that the corporation improperly obtained inside information to help win the contract by hiring two Army employes familiar with the personnel computerization project. The company and the employes have denied the charges.

The lawyers hinted at one possible line of defense by suggesting that the information allegedly obtained by General Research Corp. may also have been available to other companies that bid on the Army contract. If so, they said, the data would not amount to inside information or evidence of "criminal knowledge." The lawyers are seeking subpoenas to demonstrate their contention.

Defense lawyers also argued that parts of the indictment were unconstitutionally vague or improperly drawn up--an issue that prompted Judge Lewis to express concern. At one point, Lewis noted that no specific fraud statute was alleged to have been violated in the conspiracy count. "That's the first question I raise. What fraud section?" Lewis asked.