Ken:
I doubt that you could damage the vent as it is all tube and a snake would not go into the vent area. I also doubt that a snake would make that bend as it is a total u shape. The clog could be in the head around the pump as maybe taking the head apart might be required. You could blow it out with a garden hose, maybe, or an air compressor or the dinghy pump. Sometimes is the boat is on the hard you can work from the other side. Eventually if not of this works, you will have to take the hose fittings off...
Not an easy task.
Hayden
