Best fix is to use Corona 8, which has an improved converter. Unfortunately a lot of other materials use or allow non-physically realistic set ups, which makes it hard for the converter to know what to do with them as they have IOR set like metals, so it makes them metal. The new converter in 8 has a checkbox for "never make it metal" which is at least better in most cases :) And the converter should be getting even more love in the future - although it may still not be possible to automatically tell what a material is supposed to be in physically realistic terms, but we'll see how much better we can make it.
If not using Corona 8, you could either not convert the scene (as many V-Ray materials should render in Corona without conversion), or set things up to only convert certain things that convert well or need conversion.
