What's a culture vulture to do when you've got both the Metropolitan Opera and Frank Sinatra opening on the same night at the Kennedy Center?

President and Mrs. Reagan made the hard choice, coming down on the side of their favorite White House impresario. Other Sinatra groupies invited to join them in the presidential and contiguous boxes last night in the Concert Hall were Senate Majority Leader Howard Baker (R-Tenn.), House Minority Leader Robert Michel (R-Ill.), Sen. John Tower (R-Tex.), Sen. Barry Goldwater (R-Ariz.), incoming EPA Administrator William Ruckelshaus, former California senator George Murphy, Gannett newspapers' Allen Neuharth, ABC's Steve Bell, public relations consultant Joe Canzeri, political consultant Nancy Reynolds, presidential assistant Faith Whittelsey, Ford's Theatre executive director Frankie Hewitt, journalist Arnaud de Borchgrave, many with spouses. They nibbled sandwiches and sipped champagne at intermission.

For a select few of those who choose the opera, the night began at a black-tie dinner hosted by the Washington offices of Merrill Lynch, Pierce, Fenner and Smith. The $300,000 tour grant from Merrill Lynch's home office makes the New York-based investment firm the fairy godfather of the Met's 1983 multicity tour for the second straight year.

Says one Kennedy Center official, "If we sold every ticket in the house for every performance during the two-week engagement that still wouldn't add up to what it costs to bring the Met here."

The day may be coming when the Met will have to take fewer productions on tour, rather than the seven different operas with their seven different casts that it now takes. Only in Washington does each opera play twice.

"Since the jet plane, we've had more difficulty in persuading artists to stay on the whole tour," Anthony Bliss, the Met's general manager, said yesterday. "They don't want to spend too many weeks with one company because they have opera companies all over the world bidding for them."

Bliss returns to New York today for the opening of the Royal Ballet, then catches up with the Met again in Atlanta.

His droopy eyelids have bothered him all his life, so recently Sen. Roger Jepsen (R-Iowa) decided to do something about it. During Easter recess, he checked into a Dallas hospital for three days of eye tucks and eyelid sculpting.

"They cut out excess tissue, then stitched it back up--above and below his eyes," a spokesman for Jepsen said yesterday. "It was a hereditary condition where the tissue builds up around the eyes and could affect the vision. The senator saw it more as a potential problem than an immediate one."