Vacuum leaks can (in some cases) make an engine run lean, and that will indeed make it run hot and potentially even burn valves. Modern computer controlled systems usually adjust fuel mix for minor leaks, and set codes for major vacuum leaks that can't be compensated for. Bleeder screw does not IMHO need to be open all the way, however I have seen some cars where the valve is not located at the high spot of the cooling system such that air can be trapped somewhere.
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