The USSR never even contemplated this. There are a number of reasons.

Firstly, it wasn't necessary. Soviet analysts correctly predicted that, while it might take a long time, the USA couldn't defeat the North Vietnamese regime.

Secondly, the USSR's stakes in the area just weren't that high. South-East Asia was an area of only moderate interest to the USSR, and although they obviously did benefit from the whole of Vietnam becoming communist (and later, siding with them in the Sino-Soviet split), it was only a very mild benefit - the biggest asset that communist Vietnam provided to the USSR was access to the Cam Ranh naval base, but even that was only moderately useful, since the Soviets lacked the naval presence in the Pacific to make full use of such a distant base.

Thirdly, it would have been very difficult for them to have done so. The USSR just didn't have the USA's sealift and airlift power, and it would have been very difficult for them to support a large military force so far from their borders.

Fourthly, the Soviets knew that the USA was heavily invested in Vietnam, and during the years when the war was fiercest (from the mid 60s to the early 70s) the Soviet Union was trying to pursue a policy of peaceful coexistence with the USA. So, while they were not averse to provoking the USA in some areas, they would only do so there was a clear benefit - and supporting Vietnam wasn't such a case. Also, this was the era of the Sino-Soviet split, and support for Vietnam was one of the few things that the Chinese and Soviets could agree on - but the presence of Soviet troops in Vietnam would have probably changed China's mind.

So, basically, the costs (bad relations with the USA and China, difficulty of long range power projection) were too high, and the benefits (strategic access to South Vietnamese ports) were too low.

I'll start off with a tl;dr: Because doing that would have meant World War III.

Longer answer: There was no point in the USSR sending troops to aid the North Vietnamese. Had they done so, the American reaction may very well have been to declare war on the Soviet Union, which would have meant a nuclear exchange. To be sure, the expansion of the Communist bloc was in favor of the USSR. That is why they backed North Vietnam - Ho Chi Minh was a dedicated Marxist-Leninist who needed outside support. He did not need Russian soldiers, for the PAVN and NLF (People's Army of Vietnam and National Liberation Front, also known as the Viet Cong) had plenty of troops. What these two forces needed were guns, ammunition, anti-air emplacements, planes, tanks, trucks, and trainers for all of the above. The USSR gave them all of that in spades. It was to the Soviets' benefit to arm North Vietnam and let the US escalate the conflict - that way Moscow could bleed the US dry without sacrificing a single Russian soldier. Indeed, I would argue that the USSR was the only country that actually won the Vietnam War. They got everything they wanted out of it: a defeated US and new socialist state to fold into their sphere of influence. North Vietnam only won the war in a purely military sense. Otherwise, they more or less lost. Since their economy was in ruins, they had to rely almost completely on Soviet support, which contravened Ho Chi Minh's dream of an independent Vietnam.