"No relays, fuses, or other stuff required?"
The battery tender brand pigtail includes a fuseholder with a 7 amp fuse in it already. My gerbing jacket is 76 watts,eg, so that 7 amp is perfect for it.
All you need is to make a cable [16 gauge automotive primary wire is fine] with the battery tender style [sae] plug on one end, and your compatible style plug for the vest on the other. Make the cable long enough to suit the vest riding condition. When done riding, remove the vest adaptor cable, and you are back to a tender connection.
Get a thermostat control for the vest so you don't cook yourself.