What about talking more about the good economy? I'm sure the economy mattered to many voters in 1968. If Humphrey would have talked more about JFK's and LBJ's economic record and about how he'll continue it, I'm pretty sure he would have gotten some voters that were unhappy with Vietnam due to their desire to keep the prosperity going.
Or he could have just opposed the Vietnam War entirely.
Didn't he kinda do that on the campaign, though? He said he'd end the war as soon as possible.
He personally wanted to end the war quickly, however, LBJ threatened to hold the Democratic machine away from him, and he desperately needed the funds. So he had to walk a fine line.