The U.S. Army is one thing. "There have been jokes around here all week about who would like to tell 2 million armed employes they're not going to be paid," an official of the Office of Management and Budget said yesterday.

Not to worry. The end-of-the-month military payroll will be met.

But the Federal Labor Relations Authority is another matter. All but three of that tiny agency's 260 employes will be forced to take an unscheduled vacation without pay, possibly by the end of this week, because President Reagan vetoed a $14.1 billion supplemental appropriations bill yesterday and Congress will not reconvene until Sept. 8, after the FLRA's money has run out.

To cushion the military, OMB and the Treasury created, with the attorney general's blessing, a cash-shuffling scheme identical to one for which the Internal Revenue Service has closed countless small businesses operating on the edge. The Pentagon will delay transferring to the Treasury about $600 million in income-tax withholding and Social Security payments. That way, the Pentagon will meet its Aug. 31 payroll.

It would take only $640,000 to solve the FLRA problem, but OMB and Office of Personnel Management officials agree that FLRA is going to be the first federal agency to feel the squeeze while the president and Congress play budget hardball again.

It will by no means be the only one. Quite apart from the social programs the president described as budget busting, the veto will have the effect of slowing or even stopping the day-to-day business of government that does not qualify as nationally indispensable.

There is the possibility of furloughs for astronauts in training for the next shuttle flight, for investigators at the National Transportation Safety Board, for merit-pay specialists at OPM, for number-crunchers in the Bureau of Labor Statistics.

The Justice Department has already run out of money to pay witnesses and is delaying some trials, particularly civil actions, to save cash.

The Coast Guard will have no furloughs -- it is, after all, a military service -- but it may have to reduce environmental monitoring and fisheries management, to name two nonemergency functions.

Twenty-seven federal departments and agencies will be affected one way or another, according to the White House.

"Most of the trouble falls around the 8th, the 9th and the 10th of September ," said OPM Director Donald J. Devine. Most problems can be avoided "if Congress acts quickly," Devine said.

"I assume they have another version of this [supplemental], one that does not have those things the president doesn't want. Congress can act quickly if it really wants."

The new-found money to pay the military will carry the Defense Department only until Sept. 15, the next payday. That gives Congress seven days to work out its differences with Reagan. Most other agencies will be able to last at least that long, OMB officials estimated yesterday.

Nonetheless, several government agencies, including the National Aeronautics and Space Administration with its 20,000 employes, have sent notices to their personnel telling them that an involuntary layoff, a furlough, is possible.

"It could be as long as 10 days," said NASA spokesman Brian Duff, "although I understand the agency has been making an intense effort to minimize spending the kinds of funds that could be used for salaries."

But the "Perils of Pauline" are likely to continue, Devine pointed out. After Sept. 15 comes Oct. 1 and the beginning of the new, 1983 fiscal year. Just like last year--when the federal government "closed" for that famous day--appropriations bills for many departments and agencies are unpassed and a giant continuing resolution will have to be hustled through a presumably testy Congress in the last days of September to keep the government running.

For the moment, the only place where it is certain that furloughs of federal employes will occur is the Federal Labor Relations Authority, which is charged, somewhat ironically, with settling labor disputes between federal employes and the government.

Harold D. Kessler, deputy executive director of the FLRA, said yesterday that "based on calculations made a week and a half ago, we'll need to furlough employes for 22 work days to make up the difference between what has been spent so far and what we have.

"On Sept. 1 we are out of money. We'll have to do some calculations Monday and Tuesday to see if we picked up a day or so" with cost-saving efforts such as eliminating travel, as many federal agencies have done.

The problem occurred, Kessler said, because there has been no additional appropriation to cover last October's federal pay raise. "We have been paying people at that additional rate, hoping the supplemental would be passed," he said.

The vetoed bill included $6.1 billion for federal civilian and military pay raises that have not been covered by subsequent appropriations.

The biggest increases in the supplemental that the president doesn't want are $217 million for student financial aid; $210 million for employment of older Americans; $148 million for compensatory education for the disadvantaged, and $112 million in transfer grants from canceled interstate highway projects to other road- and bridge-building needs.

About $2 billion was cut from administration budget requests, and while much of that came from the military, almost one-fourth came from state Medicaid grants.

On Aug. 17, when House and Senate conferees were in the midst of reaching agreement on the supplemental, Budget Director David A. Stockman wrote Rep. Silvio O. Conte (R-Mass.), " . . . The first and most serious problem we will encounter is that it will be impossible to disburse the military payroll on Aug. 31, 1982, from existing appropriations."

But as a disgruntled federal employe facing a furlough said yesterday, "No one in a civilian agency had much doubt that the Department of Defense would find some way to reprogram money."