Any Soviet or Vietnamese pilot defecting to the West with ''a lethal chemical and/or biological agent'' will get a reward of $100,000 from Soldier of Fortune magazine, its editor, Robert K. Brown, is announcing today (Nov.10).The Boulder, Colo.-based magazine, which has embarrassed the CIA by acquiring various Soviet weapons from Afghanistan, is eager to learn more about Soviet chemical weapons, which reportedly have already been used in Laos, Cambodia, and Afghanistan. Mr. Brown told the Monitor that he had obtained a toxic ''gas residue sample'' from Laos. It had apparently been scraped off rocks and vegetation.The magazine is particularly eager to obtain samples of such chemical weaponry, which is said to contain deadly mycotoxins, so US scientists can develop antidotes, Monitor correspondent Stephen Webbe reports. None exists at the moment, a Soldier of Fortune spokesman said. Magazine officials are due to testify on the alleged use of Soviet toxic agents in Southeast Asia before the Senate Foreign Relations Committee today.