Stator / charging system failure
Charging-system failure — a burned-out stator, often taking the regulator/rectifier with it — is the Vulcan 900's best-known weak spot on owner forums. Warning signs are dimming lights at idle, weak starts, or a bike that dies on the road. Bring a voltmeter: you want roughly 14 volts at the battery around 3,000 rpm. A replaced stator with receipts is a plus, not a red flag.
Ask for repair records covering this item. If it hasn't been addressed and the car is in or near the mileage window, price the $400–$800 repair into your offer — or make it the seller's problem.