Should plywood be missing on my roof?
As a R.R.C. and roofer for over 25 years, this doesn't surprise me. Anyone with a pickup truck and a ladder calls himself a roofer. 
This is wrong, if they eliminated vents they should have replaced the sheathing. An opening up to 4"x4" can be covered with metal, but anything bigger requires re-sheathing to eliminate a fall hazard. Also the nail pattern should be evident. Thinking you need to call an attorney. If he did this he isn't reputable so it stands to reason he won't back up his work. Always check references and reviews of your contractor. Check with the BBB to see if there are complaints. Look at their work!! Drive by a couple of their jobs, talk to homeowners if possible. Not only end result is important but being done without damaging your property and in a timely manner is just as important.