Pipers frequently wear waistcoats (aka vests in the USA) as outer garments. They're more comfortable for piping than a jacket, and not as bad to wear when it's hot.

My waistcoat is wool front and back, and lined. It has square, silver buttons.

It's similar in appearnce to what these fellows are wearing, although it looks as if theirs might have a lining-material back. Hard to tell.

http://www.islayinfo.com/islay_pipe_band.html