Vilamoura, Portugal — After weeks of increasing uncertainty about its cohesiveness on arms control, the NATO alliance has regrouped into a united front against the Soviet Union. But its position on intermediate-range nuclear missiles in Europe has firmly nudged the Reagan administration toward ''flexibility,'' making it certain that the United States soon will offer proposals to break the Soviet-American deadlock at Geneva.
At the semi-annual meeting of the NATO defense ministers' Nuclear Planning Group, which concluded Wednesday in Portugal, officials expressed unqualified support for President Reagan's ''zero-zero'' proposal.
''This would be the best possible outcome,'' said Joseph Luns, Secretary-General of NATO. It would halt the planned deployment of 572 Pershing II and ground-launched cruise missiles in Europe beginning late this year in exchange for the dismantling of the 351 Soviet SS-20 missiles now in place.
But in their closed meetings, West European officials also were told that Washington has been mulling over ways to move toward some ''interim'' position. The group's final communique praised the fact that the US ''would continue to maintain an active and flexible negotiating position.''
The likely shape of such a proposal has been emerging from Washington and in background briefings given by those in US Defense Secretary Caspar Weinberger's party here.
Since the missile talks will recess early next week for two months, it is unlikely that a formal proposal will be offered at Geneva before then, especially since Washington has promised that it would consult its European allies before taking such a step.
The European allies agreed that any such agreement would have to conform to five principles: equal numbers of warheads on both sides, no inclusion of British and French strategic nuclear forces, global limitation (that is, including Soviet SS-20s in Asia), verifiability, and nothing that would weaken NATO's conventional forces.
Within these parameters, even the Pentagon (which has resisted moving away from ''zero-zero'') now concedes an interim proposal would be helpful and perhaps needed.
Such a proposal is likely to involve a missile figure significantly less than the planned NATO deployment of 572, and also less than the number of Soviet missiles now in place. It also is likely to require a subsequent ''build down'' to an eventual level of zero, something Moscow would have to agree to for NATO to reduce its planned deployment.
In any case, the allies have now pronounced themselves behind the initial deployment of NATO missiles if an agreement has not been concluded before the end of the year.
''If we can't reach an agreement at Geneva, then we have no option but to deploy the missiles.'' said Michael Heseltine, Britain's secretary of state for defense.
The NATO defense ministers totally rejected the latest Soviet proposal (retaining 162 SS-20s to match British and French strategic forces), noting that this would leave Moscow with more intermediate-range missiles than it had when negotiations began in 1981.