I voted 'No matter what', but a quick tuning rip around the block will forgo it. The vest is a good idea for longer cruises. People need to realize the speed these motorized bicycles get up to. Double or even triple the speed of pedaling, and you can get messed up just at pedal speed. And that's not even figuring in other vehicles...
"Better to have it and not need it, than to need it and not have it" True that Dan!
I am also with the 'always' crowd, and then some as I too broadsided a stupid motorist and went down hard.
I keep my bike parked in my home shop and have my helmet, riding gloves and shades right next to my shop door and can dawn them in mere seconds when I go for a ride, then peel 'em right back off and set them down when I get back right back there by the door for next time.
Heck, I have a couple other helmets for new customer bike pick-up test rides around the neighborhood to wear and make a huge point about safety to every one.
I am not a big fashion conscious guy, but for some of the places I ride threw and stop at I am more likely to get injured looking like a special needs kid with a bright green and yellow safety vest on than what I use to be seen by motorists...
I USE A CONTINUOUSLY FLASHING FRONT LIGHT AT ALL TIMES.
I highly recommend you do as well.
Your chances of NOT having to find out how good your helmet is in a crash are much better if you run a front flasher light too ;-}