Depends on how you define "biggest". However the Soviet invasion of Afghanistan was much smaller. The Soviets did not commit nearly as many troops as did the Americans in Vietnam, and their casualties were considerably smaller as well. (According to a quick check on Wikipedia, the Soviets suffered ~15k people killed while American killed in Vietnam were about 58k.)

In terms of impact, the American backed government of South Vietnam collapsed almost immediately after American withdrawal, and all of Vietnam became communist. In contrast, the war in Afghanistan didn't really end with the Soviet withdrawal. Soviet allies in Afghanistan lost power in 1992 - three years after, but even after that no single group was able to control the entirety of Afghanistan until the American invasion.