Vermont's 6th congressional district is an obsolete district. This district existed only during the 17th Congress. In the 13th - 16th Congresses, Vermont's 6 Representatives were elected at-large. For the 17th Congress, Vermont elected its representatives from geographical districts, but after the Census of 1820, Vermont lost 1 Representative, eliminating this district.
References
Vermont's 6th congressional district Wikipedia(Text) CC BY-SA