The French Embassy did its part last night to perpetuate the notion that there are times when Washington doesn't know whether it's coming or going.

In the case of two French ambassadors, they were coming and going. But Franc,ois de Laboulaye, who is leaving after four years, and Bernard Vernier-Palliez, who is arriving, did it with such flair that some ambassadors decided it ought to become an Embassy Row tradition.

The tip-off that the two-in-one party had been a hit in more ways than one came toward the end when the secretary of defense gobbled up three of chef Francis Layrle's addictive lemon tarts, an act regarded almost as politically significant as his presence in the first place.

"Why significant?" inquired Caspar Weinberger, reaching for a plump shrimp and downing it while being reminded that some people have been under the impression the United States is irked by recent actions of the French. The decision to sell military equipment to Nicaragua, for instance.

"Well," said the secretary, "we're not irked with a very fine ambassador who, with his lovely wife, needs to be sent off in style."

And what about the alliance, that historically imperfect relationship between France and the United States whose status a few minutes earlier de Laboulaye's successor, Ambassador-designate Vernier-Palliez, had described as "fair" and let it go at that?

"Stronger than many people seem to write about," said Weinberger, also letting it go at that.

Even in the crush of hundreds, there were those who stood out, such as administration heavies Attorney General William French Smith, Agriculture Secretary John R. Block, HUD Secretary Samuel R. Pierce Jr. and CIA Director William Casey; National Symphony conductor Mstislav Rostropovich; the Most Rev. James A. Hickey, archbishop of Washington; Irina Dobrynin, standing in for Soviet Ambassador Anatoliy F. Dobrynin, who is in Geneva; Federal Reserve Board Chairman Paul Volcker, and a lively turnout from social Washington.

Politicos, like Sen. Claiborne Pell (D-R.I.), who was made an officer of the prestigious French Legion of Honor, Sen. Charles McC. Mathias Jr. (R-Md.), Sen. Lloyd M. Bentsen Jr. (D-Tex.), Rep. Millicent Fenwick (R-N.J.) and Rep. Lindy Boggs (D-La.), and diplomats, such as the ambassadors of Morocco, Algeria, Cyprus, Egypt, Togo and Luxembourg, were en route to President Reagan's State of the Union message.

Others in the crowd said they had already gotten the message.

"Unhappily, things are getting better for the Democrats, for the wrong reasons -- the failure of this administration," said Robert Strauss, a former Democratic National Committee chairman and Carter administration official.

Said G. William Miller, former secretary of the treasury under Jimmy Carter: "The Reagan administration's first year is right on target with what I predicted. Bad."

People talked about other things, too. Like Bernard and Denise Vernier-Palliez's interest in music ("I hear he's an opera buff," said the Washington Opera Society's Martin Feinstein) and art ("He built a collection of modern art at the Renault factory," said Sen. Mathias).

Among Antoinette de Laboulaye's tennis cronies who paid respects were Olga Hirshhorn and B.J. Bentsen.

Vernier-Palliez's position as chief executive officer of the Renault automobile factory in France aroused some speculation about the car he'll be riding in as ambassador. An embassy spokesman set the record straight: the official car will be a new Peugeot 604 turbo diesel, his personal car a Renault.

In the kitchen where more pa te's and lemon tarts awaited another hello-and-farewell reception tomorrow night, this one for embassy personnel and a few members of the French community, chef Layrle wrote out some of the de Laboulayes' favorite recipes to send with them when they fly home to Normandy via Guadeloupe and Martinique. They are already planning a return visit.

"We've been friends a long time," said Bernard Vernier-Palliez. "Antoinette and I went to dances and used to ski together. That was before she married Francois, of course."