I just found a number and called the assistant state fire marshal. He told me the clearance in vermont for single wall stovepipe is indeed 18 inches. He told me I should find the section in NFPA-211 and tell them I checked that document and under section bla bla it states 18 inches is the required minimum clearance. He said then they will know I've done my research and should do something about it. Oh how I love making phone calls like this.