The town of 'Salem's Lot the proper name for it is Jerusalem's Lot - the locals shorten it in the bigger novel. The only reason that I don't like to view it as a prequel is the location. In this the town is right near the sea, Chapelwaite is on the cliff edge. Whereas 'Salem's Lot in the bigger novel is nowhere near the sea.
Maybe... we also don't know the extact distance between Chapelwaite and Jerusalem's Lot. In this time Jerusalem's Lot was a forgotten and destroyed village, in the main novel it was already a normal American little town. An idea could be Chapelwaite would the location of the later Marsten house... And in the short story "One on the road" Jerusalem's/Salem's Lot was already forgotten again... And what matter with the unholy church in Jerusalem's Lot for Salem's Lot?
