Yes, said the security man, new locks had been installed on the room doors. Yes, said the credit woman, checks counter-signed by certain VIPs would be accepted. Mais oui, said the banquet manager, 2,500 people could be served filet mignon with all the trimmings in an hour.

Thus is a Washington convention born. It is planned to the teeth and doublechecked to distraction. But it is always hovering on the edge of disaster. The only way to avoid it is blood, sweat and tears - extra quantities of them, too, especially when total attendance verges on 2,000, the capacity of the largest Washington hotels.

That was the approximate turnout at last week's annual midwinter convention of the Veterans of Foreign Wars. It was held at the Sheraton-Park Hotel. Both institutions survived the experience, but they never would have if, three months earlier, Cooper T. Holt had not picked up a phone.

Holt, executive director of the VFW's Washington office, is a cheroot-smoking Tennessean who spends most of the rest of the year lobbying on lofty or momentous matters, such as war and peace.

But five days before his first convention delegate arrived, he was in a Sheraton-Park meeting room, grilling a hotel executive in dead seriousness as to whether the piano in the banquet hall had been tuned.

"It may sound like chicken-feed," said Holt, who actually used a similar, but more earthly, noun. "But we know we've got to be careful. Our people are always very impressed with our conference, and we aim to keep it that way."

So does the Sheraton-Park. The VFW's midwinter gathering is the oldest repeating convention the hotel has. This year's was the 22nd in a row.

But just because they run the hotel does not mean the S-P staff runs the convention. They organize, execute, critique and check the master plan. But Holt and 30 people on his staff are the ones who compile it.

Coordination is constant, across three months. Holt estimated that he and his staff sent 50 letters and made "thousands" of phone calls to the hotel in battening down this year's festivities.

It all comes together at a final planning meeting, called a "tie-down."

Wags say it got its name from what they had to do once to a hotel manager who had heard requests for 8,000 standby bellhops once too often. But realists say the "tie'down" is the meeting that makes a convention work.

Nord Schwiebert, managing director of the Sheraton-park, opened this year's "tie-down" with a deep slug of coffee and a first-name greeting to Holt. Then he introduced 12 of his key staffers, who took turns ironing out late wrinkles in their areas of specialty.

But it was in Eli Vidnjvich's bailiwick that the planning got down to the finer points.

Vidnjvich's title is Director of Function Arrangements for the Sheraton-Park. But he is really the professor of nuts and bolts. In order, he assured Holt that:

A three-foot-lon'ramp, covered in blue carpet, would be provided for Veterans Administration Administrator Max Cleland, a triple amputee who uses a wheelchair.

There would be American flags in every meeting room.

The spotlight operator would aim at the big American flag on the back wall just as soon as John Wayne's pre-recorded patriotic welcome to the banquet was over.

And never fear, the piano had been tuned only a few days earlier.

The biggest problem, to Holt and his three assistants, was inducing the Sheraton-Park to agree that when necessary, it would hand out room keys to arriving delegates before the rooms had been make up.

No hotel likes to force a guest to confront someone else's dirty towels. But the VFW likes it less when hundreds of delegates are milling around the lobby or bar, rapidly getting separated from their bags and even more rapidly getting mad. The hotel agreed.

Small problems abounded. Could a second cup of coffee be served at the banquet without its running past 9:15? No? Then forget the second cup. And could the VFW's housing man, Art Donahue, stand behind the front desk to facilitate check-in, as he had in the past? Yes? Fine.

The "tie-down" ended in cherry diplomacy. "We're looking forward to a real smoothie," said Holt. "You shall have it, sir," said Schwiebert, extending his hand.

Smoothiest are far from the norm in Washington, even though conventions are among the biggest of businesses here.

In 1977, according to Austin Kenny, executive vice president of the Washington Area Convention and Visitors Association, 825 conventions brought 715,000 delegates here.

The delegates followed-much the same pattern as their counterparts of earlier years. They paraded around in "Hi, My Name Is . . ." buttons. They called their waitresses "hon." Doubless, they tried, and failed, to figure out Dupont Circle.

Most significantly, however, they and other conventioneers brought $214 million with them that they didn't take home.

But according to Kenny, the area's lack of a convention center jeopardizes the future of such figures. Without a center, large Washington conventions must often be fragmented among several hotels. That may keep cab drivers business, but for smoothies, many groups say give me Anaheim instead.

Not Cooper Holt, however. Since his midwinter conference centers on political and legislative matters, Washington is a must location. But he said he would choose it anyway.

"When you see all the planning and promoting coming to an end, it always makes us very happy," he said.