Bangkok, Thailand — ''The Vietnamese are the most difficult allies imaginable,'' groaned a Soviet official. ''You never know when they're going to take offense at something.''
''The Soviets are nice, but pushy,'' a Vietnamese official remarked. ''You have to know when to smile and agree with them, and when to scream and kick.''
The Soviet-Vietnamese relationship is an odd one: close but uneasy. Both sides know they can't do without the other, but clearly wish they could.
According to Western sources, Moscow gives Hanoi about $1.1 billion in military aid every year, including most of the weapons for its 1-million-man army. Since 1951, 15,000 Vietnamese have studied in the Soviet Union. Economic aid since 1978 is estimated at some $3 billion.
But if Moscow is generous with its aid, it is also free with its criticism. Hanoi's last five-year plan was a disaster, and the Russians are bitter at the waste of their money.
''Our aid is very big,'' says one Soviet official. ''If the Vietnamese had used it properly, they would have made much more progress over the last few years than they actually have.'' The aid was, he says, squandered due to ''incompetence and ignorance.''
Despite these harsh words a flurry of agreements signed last year commit Moscow to heavy support for Hanoi's 1982-86 plan.
Why does Moscow bother? The Vietnamese have a ready answer. ''The Russians benefit enormously from their alliance with us,'' says one official. ''Without it they have no influence in Southeast Asia. And the alliance keeps China worried. We, though, pay an enormous price for the friendship - continuous Chinese sabotage on our northern border.''
The Soviets also benefit from access to port and airfield facilities at Da Nang and Cam Ranh Bay. But despite their massive aid program they have been unable to wheedle out of Hanoi the sort of base agreement that the US has with the Philippines.
Until 1978 when relations with China finally collapsed, Vietnam strove to keep some sort of balance between the two communist giants. When alliance with Peking turned to enmity, Hanoi, perhaps unwillingly, joined COMECON and signed a friendship treaty with Moscow. Vietnamese officials admit, however, that some Vietnamese communists are still having difficulty accepting this relationship with the Soviets.
''We don't have any problems with the top leaders,'' says a Soviet Indochina specialist, ''but there are still some second echelon leaders with Maoist tendencies: They still want to distance Hanoi equally between us and Peking.''
Should Vietnam ever try to do this, he hints, it would have to be prepared to pay the price in reduced Soviet assistance. ''Our aid effort, after all,'' he says, ''requires very close collaboration with Hanoi.''
Speculation about Vietnamese-Soviet friction has most recently centered on Vietnamese-occupied Kampuchea (Cambodia), where some Western observers suggest that Khmer party chief Pen Sovann's downfall was due to his overfriendliness with the Soviets.
Hanoi will not tolerate any attempt to weaken the special relationship with Cambodia that it considers paramount to its own security. Pen Sovann and other leaders are thought to have been chafing at the closeness of the relationship, and may well have approached the Soviet Union for help.
Some anti-Vietnamese Khmer leaders are also thought to have tried to establish economic relations with Japan in an effort to loosen Hanoi's grip on the country.
While he was trying to move away from the Vietnamese, however, Sovann seems to have made the mistake of alienating his own Khmer colleagues, who are thought to have been deeply unhappy at Sovann's power monopoly.
Unless Vietnam reveals details of some hitherto undisclosed anti-Vietnamese plot by Sovann, the most likely explanation for Sovann's downfall is that Hanoi decided to capitalize on the internal divisions of the Khmer party by overthrowing -- or allowing the overthrow of -- a party leader who was as unpopular with Khmer leaders as he was with Hanoi.
How far the Soviet Union would have gone in backing Sovann against Hanoi is open to question. ''We couldn't control the Vietnamese in Indochina even if we wanted,'' protested one Soviet official. ''Besides,'' he added, ''Pen Sovann's problem wasn't that he was pro-Soviet -- he just wasn't pro-Vietnamese."