Two former secretaries of defense -- James R. Schlesinger, who served under President Ford, and Harold Brown, who worked for President Carter -- expressed serious reservations yesterday about the Reagan administration decisions on future strategic nuclear forces, epecially plans for the MX missile.

"It's decision to putn," aid Schlesinger of the plan to install the MX initially in remodeled and strengthened underground silis now used for old Titan II missiles.

"It does not deal with the vulnerability issue that has been advertised at considerable length. By itself," he said in a telephone interview, "it does not close the 'window of vulnerability." It's a decision to go ahead with the missile and look around for a way to protect it later on."

"I guess it's better than nothing, but it's not much," said Brown in an interview. "Clearly it does nothing to solve the vulnerability, which I consider to be real."

The vernerability issue involves claims by the Carter and Reagan administrations that Soviet long-range missiles were becoming accurate and numerous enough to wipe out all but a small percentage of the 1,052 land-based Minuteman and Titan II missiles is underground silis at fixed locations in the western United States.

The Carter administration, expanding upon ideas developed during the Ford administration, decided the best way to close the "window" was by deceptively basing the MX, shuttling 200 missiles among 4,600 shelters in the southwest in a huge shell game to confuse Soviet targeters.

The Reagan administration rejected that plan, claiming the Soviets can make MX instantly vulnerable just by building more warheads to hit all the shelters, which are not as strong as silos. Instead, the Reagan plan is to put 100 of the long-delayed missiles into production so that their powerful and accurate punch -- each mX carries 10 warheads -- can at least be added to the arsenal while research is accelerated to find a better and more permanent way to protect it.

The first 36 missiles and maybe more, however, will go into fixed Titan silos, with greater so-called "hardening' for added protection, and that decision has stirred instant controversy.

Brown says the administration "is just whistling in the dark" when it comes to trying to harden existing silos with confidence. Brown and others are known to have doubts about the degree to which existing silos are able to withstand the enormous blast and pressure from nearby atomic explosions. Schlesinger also suggests that the Armed Services committees on Capitol Hill take a hard look at how the hardening plan would work and what it would cost.

Some defense specialists raise questions privately about whether the White House looked deeply into the question of how to harden the Titan silos, because the decision to do so was known only to the president's top aides.

Schlesinger says his chief worry about the overall $180 billion Reagan strategic package is that it spreads around resources and may run up costs. "This is a program of sizable costs while the military is under severe pressure and does not seem to achieve the goal of closing the window of vulnerability. One would want to make sure it does not result in draining away resources from general purpose force," he said.

Retired general Maxwell Taylor, a former chairman of the Joint Chiefs of Staff, interviewed on the Cable News Network, said yesterday that he also was concerned about the expense because the more urgent need is to beef up conventional forces.

The administration has stressed that its strategic program will take less than 14 percent of the defense budget and that doing away with the shell game also saves money, at least initially.

Thought Brown says there is at least some value in getting the MX into the field and leaving open options for the future, he believes that the overall decision "is a mistake" and that it virtually assures that none of those longer-term options could be available until 1990 at the earliest. He also feels that Moscow will probably continue to build up its warhead arsenal anyway and that, without the MX shell game, the Soviets will be ble to barrage many other U.S. military targets more heavily.

Edward N. Luttwak, a Georgetown University professor and strategist who served on the Reagan transition team, considers MX to be a "flawed system" but also better than nothing, considering the urgency of the need to rebuild strategic forces. He argues that the only answers to land-bases missile vulnerability are either effective anti-ballistic missile defenses or a much smaller and more mobile missile that can be easily moved around the country and be hidden.

The Reagan administration has not tried to advertise the MX-Titan decision as anything more than an interim solution, acknowledging that it didn't know a better solution yet but would try and find one by 1984.

Richard Perle, an assistant secretary of defense for international security policy, said in an interview that what we are seeing now is a fundamental change from periods when the United States could build new strategic systems with considerable assurance that the Soviets would not be able to quickly knock them out. "That simply is not longer the case," Perle said, "and we are now looking at a series of partial solutions," each of them perhaps inadequate by itself but when taken together greatly complicating any Soviet plans.

Thus it is conceivable, defense officials say, that years from now MXs might be deployed on airplanes, in old silos and in new ones.