Metal panels over underlayment directly over shingles?

I am having a metal roof installed on our house. The house currently has 3 tab asphalt shingles that are 13 years old. They are in relatively good shape, but we had a crazy wind a few months ago and it has displaced some of them. One of the contractors is wanting to install a vapor barrier underlayment and then the 29g roofing panels directly on the underlayment/shingles. He says he uses 1.5" screws that go through the shingles and into the roof decking. Another contractor is saying doing it this way WILL cause leaks and they will back out. I'm curious if one way is better, if one way is definitely NOT recommended? Thanks in advance!!