Actually Nixon negotiated a successful withdrawal from Vietnam from a position of strength, and left the South Vietnamese in control. It was the Democrats controlling Congress (and their allies in the press) that caused the mayhem later when the Viet Cong began violating the negotiated treaty. They tied Nixon's hands and refused to fund any military support to force the Vietcong back across their lines.
The obvious lack of US support or intent to enforce the treaties cause an internal fear and a self-fulfilling collapse on the part of the South, as everyone turned tail and ran.
Watergate hurt Nixon politically, but it was the very partisan Congress and the press that caused the US to abandon Vietnam after fighting to a near victory. The abandonment of the South Vietnamese would have happened with or without Watergate.
Read it in Nixon's own words. One of the best books on Vietnam ever written was Nixon's book, "No More Vietnams." I'm sure its available on paperback through Amazon or elsewhere.
I've read all of them. '72 was also an election year for the Congress. Without Watergate looming, he might not have had to deal with so lopsided a Congress. He might have had the confidence and, yes, the time, to campaign for Republicans. Shirt-tails count, and Watergate--just 5 months old and in the papers nearly every day--cut them off.