Worm I don't have the vast knowledge base to rely on like the rest of you guys but my gut feeling is no, they weren't built in N. Vietnam.
The only thing I can base my argument on is that in the early years of the war the VC were using tons of weapons/ammo from the WWII era from the allies and axis powers. We have no sound evidence that I'm aware of that the N. Vietnamese built factories/armories to produce the SKS. Even if they tried and possibly even succeeded in making a few I believe the effort never got off the ground in a notable scale because of the extensive and devastating bombing campaign of the north. If this were the case, what would one of those rare carbines be worth among collectors today?!
We know that China and Russia sent enormous amounts of aid in money and weaponry, also man-power. I have met Veterans that said Russian soldiers were occasionally spotted in the field in the mid to late 60's. During this period I do not think it would have been worth the effort of N.V. to manufacture their own weapons when they were receiving everything from anti-aircraft missiles/guns and armored vehicles on down to the AK47 in large amounts from the Chinese & Russians. It just doesn't seem logical nor economically feasible in my view.
Having said all this, I'm going to duck down and button my hatch now!