I think you've lost me King.

The Khmer Rouge were Cambodian communists who took over power in Cambodia AFTER the Vietnam war. The North Vietnamese army with assistance from China (mainly weapons) are the ones who finally captured Saigon, forcing the South's surrender and re-uniting the country under the northern communist rule.

To my knowledge, the VAST majority of the southeast Asian coastline is shallow swampland. If the Soviets actually wanted a southern port, they could have just invaded Thailand or Cambodia which would have given them already established ports in the gulf of Thailand.

Soviet involvement in the war WAS NOT to gain more territory! China would NOT have allowed that to happen. Besides, any kind of meaningful supply routes over land from the U.S.S.R. to the gulf would have to cross Chinese territory, unless they went OVER the Himalaya's through India, which again is HIGHLY unlikely.

The Soviets supported the global spread of communism and the downfall of western democracy, that's why they supplied arms and advisors to North Vietnam.

p.s. Here's a map of southeast Asia. I'm pretty sure there are NO high granite mountains surrounding the Gulf of Thailand (though this map isn't very detailed) to provide what you alledge.