Democrat Avery Vise has ended his longshot bid for Congress, in Alabama's heavily Republican Sixth District.   In a letter to Alabama Democratic Party Chairman Nancy Worley, Vise said he has business opportunities that could require much of his attention.  He also said that he tried to recruit a replacement candidate, but was unsuccessful.  The decision means that Republican nominee Gary Palmer currently has no major party opposition in the November general election.   Democrats have two weeks to find a new candidate.