I agree with the others who have said that clothes must fit properly to be flattering--vintage or not. I also think a man has to be comfortable and confident in what he's wearing in order to come across as sexy. There's nothing like fidgeting with your outfit to ruin the look. While my husband wears jeans and a t-shirt with a button-down almost every day, he's comfortable in a suit. When we go to formal events and I see other guys squirming because they don't feel right dressed up, it makes me appreciate that I have a man who can carry himself well no matter what he's wearing.
But if he's comfortable and confident in vintage, that's a bonus.