The Snorkel Vest is like the Horse Collar Buoyancy Compensator but without the bells and whistles. Mine has dump valves, button inflator, manual inflator, pockets, gear rings, etc. I use it for snorkeling and SCUBA. I like to get my weights adjusted so that I am very slightly negative buoyancy on the surface with no air in my BC. A small puff or squirt of air and I'm neutral.
If I'm snorkeling, I'll adjust the air in it so that I'm just barely floating on the surface, "awash" as the description goes. It's still fairly easy to submerge that way but if you want to go deeper or stay longer, dump the air from your vest.