I'd rather keep Dubie at Lt. Gov. than have him run for the House (even if we assume that he wins the House race).
And if Sanders would "dominate" a three-man race with Dubie and Racine (as in, have a lead of 8%+ the whole way through and deliver on election day), then I don't think that Dubie would win the House race either. Doug Racine was the Democrat gubernatorial nominee in 2002 (not 1976 or something) and I don't see how he would get below 20% or so of the vote (especially with a D nest to his name), so if Sanders wins by 8% he would get 44% to 36% for Dubie, and if Dubie can't get over 36% against Sanders and Racine I don't think he'd get 50% in another federal race.
I would say it has very little to do with Dubie and everything with Sanders being the de-facto incumbent in the Senate race vs. the truly open House race.