The short answer is "The tip should be set to positive (+).
I found an old Vector VEC127Y Spotlight. It appeared to be in perfect condition but the battery was dead and there was no charger. I was able to buy a replacement battery at BatteriesPlus for $21 USD. Here is the link for the battery REPLACEMENT VECTOR VEC127Y BATTERY
I also purchased a Universal A/C adapter to use as a charger at BatteriesPlus for $17 USD. Their site does not list it yet but here is the manufacturers link. Please note the original battery was rated 6Ah. The replacement is only rated for 5Ah. Which means one less Amp hour (Ah). Which means a shorter run time, after charging, than the original battery had. Otherwise they are equal.UNIVERSAL AC ADAPTER
I installed the new battery. I set the AC Adapter to 6V and installed the largest of the seven adapter tips that came with the charger, with TIP set to be positive (+). I opened the battery compartment of the spotlight and disconnected both the red (positive +) and black (negative -) battery leads from the battery. I then plugged the adapter tip of the Universal AC Adapter into the spotlight and plugged the AC Adapter plug to a power outlet. With a multimeter I tested the polarity of the spotlight battery leads while charging. I was able to confirm that the Adapter TIP should be set to positive (+). After reconnecting the spotlight battery leads and setting the AC Adapter to charge, I have a perfectly great spotlight in full working order.