MURDER AT THE VICARAGE - At Olney Theater through June 24.

After 26 years of making it worthwhile for Washington theatergoers to drive all the way out to Olney from June to early September, Olney Theater is doing something unforgivable. It is behaving like a summer theater.

Olney has been summer theater before only in the narrow, technical sense of being a theater that operates in the summer only. When it started, the Olney area had no townhouses or 24-hour-a-day grocery stores, and thus was cooler than the city. Those Washington summers, when almost no one had private air-conditioning, movie theaters were always filled, often with people who hadn't bothered to check what was playing.

But it wasn't necessary to compromise with quality if one went to Olney. There, unlike the standard summer-theater circuit, one did not have to suffer through slap-dash productions of chestnuts of plays.

However, that is what this year's opening play is. Agatha Christie plays have been Olney staples and sell-outs for years, but previously, either the plays or the productions or both were snappy and sharp.

"Murder at the Vicarage" is none of the above. The tedium of having everyone immediately say something nasty about the same - person thus clueing in the audience that he is as good as dead - and then having each one say one thing fishy and one thing touching about himself, if not relieved by humor.

To other such plays, Christie added sparkle, or, at Olney, the actors pitched in with the necessary verve. This time, the cast, directed by Leo Brady, can hardly cope, let alone get things bubbling. Opening night, several actors muffed their lines. References are made to the sexiness of characters who seem ludicrous or unpleasantly slimy. Clues are recited so boringly that one has to force oneself to listen.

Some actors who are known to Washington audiences from many excellent performances, contribute to the lack of success - the only person who brought any sense of liveliness to it was Nancy Nichols, as the vicar's young wife - as do some newcomers. One can only wish everyone a better rest of the summer.