The problem is, you need to have a little bit of flex in the coupling. I think that is why a lot of people use the vinyl. The metal couplings I've seen have those slits that go pretty deep into the part so the coupling can flex a little bit.

These aren't quite the shape I'm talking about. The ones I'm thinking of have slits straight across instead of in a spiral shape:

http://www.ebay.com/itm/5mm-To-5mm-F...item4d1aad5ed8