Here is a useful (if somewhat difficult-to-parse) comparison, from 2005, of public opinion on Vietnam versus Iraq; the Vietnam data starts in August 1965; the Iraq data in March 2003:
After the Tet Offensive, the percentage of the public saying that the Vietnam War was a mistake rose from around forty percent toward fifty percent, reaching 53 percent in August of 1968 (If James Joyner's analysis of this data is right) and continuing to rise slowly throughout the Nixon years. (However, the "mistake" numbers for Vietnam didn't reach today's "Iraq was a mistake" levels until around 1973, five years after the first Nixon campaign). So the Iraq War will be about 5-10 percentage points more unpopular in this election than the Vietnam War was during the '68 race - assuming, that is, that the Iraq War numbers don't worsen (or improve) in the next six months.
Update: More thoughts from Jonah here.