Despite few recent purchases, most of the Vietnam military equipment is Soviet r Russian origin. If Vietnam gets in conflict with China, Russia might not want to provide resupply. Partnership with China might become more important than relations with Vietnam. That would leave Vietnam in difficult position. The possible conflict might be about South China Sea, It has happen in the past. When Somalia went to war with Ethiopia, both countries were allieas of Soviet Union. However, Soviets decided that Ethiopia is more important. Somalia lost a war as a result, and that started the chain of events which turned Somalia into model of failed state.

Russia like Vietnam eventually will fall in line with the Yuan currency pole opposing the Euro and Dollar pole. The Yuan will be backed with gold, so there is not much that Russia can care about Vietnam indeed, let alone if Vietnam later on starts buying oil priced in Yuan from China or Russia.

Yes, Russia is sliding deeper and deeper, inch by inch, into Chinese "embrace". Not that they like it, Russia is not rich. Soviet Union wasn't, too, just like imperial Russia before. There is no shame in that, most countries aren't. Vietnam is something else. They had been hostile to Chine for centuries. The only period when they were not was when both countries were communist (technically, they still are). However, they went to war in 1979 despite "brotherly" relations. Vietnam will join whatever association or alliance they have to, just to break away from Chinese domination.