This short-sleeved knit vest originally had a plain, large plastic button for closure, at the neckline.
I removed that button and replaced it with the female half of a large snap. (I now know where to find the same size snaps in black, but fortunately for this posting, these are silver and easy to see against the black knit.)
Having whip stitched the buttonhole closed, I sewed a male snap part to the middle of the backside of the closed buttonhole. At that point, the vest could be snapped together, but had a blank, buttonless front.
Next, I sewed another female snap onto the outside, over the sealed buttonhole.
The last step was to sew a male snap part onto the back of my Dancer button and snap it onto my garment.