I think Byte Me has a lot of good information.
Your question seems to have a mistake. The VC are NOT the NVA. And the USSR did not officially supply the Cong, but they did sell weapons to the NVA because the NVA were part of the NV government, got it? So NVA weapons were Soviet made. The VC weapons situation was quite different.
The VC were a guerrilla group. They got weapons wherever they could, and they had trouble doing so many times. They improvised. I am sure they made use of these:
1. Ordnance left by Japanese after WWII.
2. Ordnance left by the French in the 1950s.
3. Ordnance supplied by the NV government, sent to them by the Ho Chi Minh trail.
4. Ordnance supplied by the Chinese.
5. Ordnance stolen from American forces.
6. Ordnance obtained on the black market from Americans.
7. Improvised weapons and devices.
8. Weapons captured or acquired from ARVN.
Against the Americans, they basically started with 1,2, and 7. By about the late 60s, their main supply switched because the war was expanding quickly and all of the old stuff was getting used up. New recruits needed new guns. Luckily, some new sources came on line, 3, 4, 5, 6 above. Of those, 4 never became very significant. 3 was important mostly for heavier weapons in the late 60s. The problem became so bad, with VC attacking with heavier and heavier weapons, that LINEBACKER carpet bombing and defoliants were used more and more. The Christmas bombings were also intended to keep NV from helping the VC.
Well, NV was running out of money and the USSR was kind of losing interest itself... they never were rich, after all, and China was no great friend, so by about the time of the Tet Offensive, the VC were getting most weapons from 5, 6, and 8. We kept shipping weapons over, and they kept disappearing. Corruption was pretty bad in Saigon, as you can imagine.
By the time the Americans left, they were leaving weapons to the ARVN, basically abandoning it in many cases, and the ARVN was unable to use it or manage it all. By 1974, after the Americans had gone, it was easy for the USSR to send in tanks with the NVA to finish the job, but the VC were extremely pissed off. They had been winning with very little help from the NVA or anybody really, and here were these tanks with NVA soldiers in 1975 coming in to plant the flag at the US embassy.
They are still pissed off. All you have to do is ask them. The VC very cleverly kept themselves supplied and winning, and the NVA came in and hogged the spotlight at the end.
They had enough weapons left over at the end of the war that they were able to fight wars with China AND Cambodia using leftover American ordnance. To this day Chinese people believe that the US somehow induced Vietnam to attack China. Why? Because all of the Vietnamese weapons were American.
Government documents available at projectgutenberg.org . VERY dry reading.
Personal conversations with professional people in Vietnam and China.