President Reagan won the first vote of the year on the MX missile, as a House Appropriations subcommittee yesterday approved a resolution to release $1.5 billion frozen by Congress last year for an additional 21 missiles.

The 7-to-4 vote in favor of releasing the funds was expected -- the defense subcommittee has supported the MX in the past -- and is only the first in a series of votes required before funds can be released.

But opponents of the missile acknowledged that it was indicative of the uphill struggle they face in trying to defeat the MX in the midst of renewed arms control talks with the Soviets.

The administration has been lobbying hard for the last two weeks to win release of the funds, and Reagan has said continued funding of the MX is crucial to the arms talks, which opened yesterday in Geneva.

MX foes said they expect to make the case in the next few weeks that the MX is not vital to national defense and that it is too costly in a time of sharp budget cutbacks.

"The president is clearly advantaged by having the MX votes in this time period," said Rep. Richard A. Gephardt (Mo.), chairman of the House Democratic Caucus and an MX critic. "But at the same time, the budget issues have started to come more clearly into focus."

In the subcommittee yesterday, MX supporters argued that the administration should not be denied a weapon that could be a "bargaining chip" in the arms talks.

"The place to have this issue decided is between our negotiators and those for the Soviet Union, not here," said Rep. Joseph M. McDade (R-Pa.), a sponsor of the resolution to release the $1.5 billion. He said that denying the administration the additional 21 missiles "would be devastating" to the U.S. position at Geneva.

But MX opponent Rep. Les AuCoin (D-Ore.) said the missile is too vulnerable -- "a glass jaw in our strategic forces" -- and "totally irrelevant" to the arms talks. "After we funded the MX in 1984, the Soviets walked out of the arms talks. After we voted to fence freeze the money, they came back," he said.

The 7-to-4 vote in favor of the MX was similar to the panel's 9-to-4 vote last year. In both cases, the four lawmakers voting against the MX were AuCoin, subcommittee Chairman Joseph P. Addabbo (D-N.Y.), Appropriations Committee Chairman Jamie L. Whitten (D-Miss.) and Silvio O. Conte (R-Mass.).

Addabbo said yesterday that the resolution is likely to come before the full committee next week. That panel has backed the MX in the past, although by only a one-vote margin last year. Opponents and supporters agree that the real fight over the missile will occur on the Senate and House floors.

Under a complicated arrangement worked out last year between the Republican-controlled Senate and the Democratic-led House, the MX legislation will be sent to the floor of both chambers regardless of what action is taken at the committee level.

Under the arrangement, the $1.5 billion for the 21 missiles was approved but could not be released unless Reagan requested and won new votes on the issue, no earlier than this month. The president last week formally requested the new votes.

To get the funding released, the administration must win two votes in each house -- first authorization for 21 more missiles and then a $1.5 billion appropriation.

Although the House subcommittee took the first MX vote of the year, congressional leaders have agreed to bring the issue to a floor vote in the Senate first.

If the Senate kills the MX, there will be no need for a vote in the House, whose Democratic leaders are skittish that an anti-MX vote in that chamber will add to the Democrats' image as "soft" on defense issues.

MX opponents are hopeful that they can win in the Senate, which approved the MX last year when Vice President Bush broke a tie. After the November elections, the Republican majority in the Senate was cut by two, and MX opponents say they think that they now may have 51 votes against the missile.

In the House, MX opponents began mobilizing yesterday, with the support of the Democratic leadership, which had appeared hesitant last week to take on the president on this issue, in part because some of the more conservative Democrats had raised concerns about the party's image on defense.

Meanwhile, at another House panel yesterday, Soviet defector Arkady N. Shevchenko said that a vote against the MX would not "make or break" the arms talks and that he thinks Soviet leaders are more concerned about Reagan's proposed space-defense Strategic Defense Initiative.