The US had over 86,000 casualties in Vietnam in 1967. So the Russians aren’t doing that much worse against a much better armed and supplied force.
“The US had over 86,000 casualties in Vietnam in 1967.”
That was over a year, at the height of the war, not the first six months.
By that point in the Vietnam War, Russia and China were sending 300 tons of weapons and supplies per day down the Ho Chi Mihn Trail. Over 15,000 Soviet and 300,000 Chinese communists were serving in North Vietnam, including piloting combat aircraft against American forces. Thousands of howitzers had been provided, and millions of Artillery rounds.