Last time I checked, southeast Asia in terms of terrain, political situation and proximity, is far different than afghanistan.
Just saying.
Terrain may be different, but it's no less shit. The Afghans have mountains to hide in. The Vietnamese used the jungles, even those of their neighbours.
As for political situation, since you're looking at how the Soviets would deal with the Vietnam War in the shoes of the Americans, you'd except nationalism to bind Vietnamese as strongly as Islam had with the Afghans. It's what drove the Vietcong (the US just seemed like the more obvious puppeteers with Saigon than the Soviets had with Hanoi, at least to them).
Map-wise, the Soviets did seem like they're closer, by raw distance, but you have to mind that most of their important stuff are on the European side, not in the Far East. They'd had to drive supplies via the European heartlands through Trans-Siberian Railway, then through the railways of China or ship them out of Vladivostok. Hence, it's nowhere near simpler than the logistics the US had to face in the Pacific. Hell, Afghanistan was
right at their border and they're having a hell of a time.