The unexpected visit to Moscow last week of a Vietnamese delegation underlines once again the uncomfortably close relations between the Soviet Union and its rather feisty Southeast Asian ally.
''Whenever in our 4,000-year history Vietnam has been dependent on one large friend, it has been a disaster for us,'' remarked the Vietnamese premier, Pham Van Dong, in 1978. When Dong made that comment, however, Vietnam was being pushed toward one large friend - the Soviet Union. The Chinese had cut off aid, and the United States was rebuffing efforts at diplomatic normalization.
Since then Vietnam has beentighPly tied, at least financially, to Moscow's apron strings. And the subject closest to VieTnamese hearts has been Soviet aid.
Western sources generally estimate Soviet aid to Vietnam at $3 million a day - a figure that many people quote, but few would like to try to substantiate. What is perhaps more striking is that Moscow has not yet announced an aid package for the current Vietnamese five-year plan. Some leaders have hinted that they expect aid to be larger than the amount given for the previous five-year plan, when the Soviets provided an estimated $3 billion.
The current plan is rapidly nearing the end of its second year, however, and Hanoi is beginning to get worried.
The delegation to Moscow, led by Vietnam's second in power, Truong Chinh, arrived at a bad time to discuss aid. Soviet President Leonid Brezhnev referred in his talks with the Vietnamese to ''considerable problems and deficiencies'' in the Soviet economy. Signs were just emerging of another bad Soviet harvest - the fourth in succession.
Truong Chinh does not seem to have been moved to pity. The Soviet Union, he said in a speech, had become ''a leading industrial country'' - and one that always fulfilled its ''international obligations.'' And he stressed again that Soviet aid was vital to Vietnam's current economic plan.
Despite this urging, the Vietnamese were able to come away with no more, apparently, than a commitment in principle to continued aid - an embarrassing end to such a high-level visit.
Vietnam's economy is still in a terrible state, although new economic policies have resulted in improvement in food production this year. Consumer prices doubled last year, according to the International Monetary Fund, while imports grew and exports declined. Late in 1981, the IMF says, Vietnam's foreign reserves stood at $16 million - not enough to cover one week's imports - while the country's foreign debt stood at $3.5 billion, $2.2 billion of it to the East bloc.
Hanoi's attitude toward Moscow's overtures to China seems somewhat ambiguous. But the move is a reminder that Soviet and Vietnamese interests often diverge.
In 1972, for example, while Haiphong harbor was mined, the Soviets discussed detente with Washington. In 1979, a few months after Peking attacked Vietnam, the Soviets proposed bilateral talks with China.
On the other hand, the Vietnamese have themselves been proposing, unsuccessfully so far, talks with Peking. In the long term they would probably be quite happy to see a return to a rather more evenly balanced relationship with the two communist powers - one where both Peking and Moscow bid for Vietnamese friendship.
During the talks, the Soviets had their own gripes. The perennial problem of inefficient use of aid came up again, accompanied by an unusually frank signal from the Soviets that they are tired of footing the bill for Vietnamese intervention in Kampuchea (Cambodia). The cost of maintaining Vietnam's estimated 160,000- to 180,000-man army in Kampuchea has never been revealed. As the ordinary soldier receives 28 dong ($3) a month, however, the annual salaries for 160,000 man force would add up to at least 54 million dong ($5.9 million at official rates). Hanoi's economic aid to Phnom Penh in 1979 was $56 million, while Vietnamese officials say that the treaty of friendship with Kampuchea commits Hanoi to at least $25 million in economic aid yearly.
All this adds up to a big burden for a poor country.
Moscow's main strategic0interest in Vietnam seems to be the massive naval and air base at Cam Ranh Bay built by the United States. Soviet naval vessels call regularly for refueling at Cam Ranh, while four Soviet Tu-95 strategic bombers are said by Western sources to be stationed there. The Soviets have long pressed Hanoi for permanent rights to the bases. So far the Vietnamese have remained adamant: ''We won't have foreign bases on our soil,'' said one Vietnamese official, ''it's question of principle.'' This year, however, the Vietnamese foreign minister began to hint that the decision might possibly change.