A lot of Vray-specific maps (like VrayDirt or VrayColor etc) are already converted automatically. Not all of them of course since there's not always a way to convert it to something useful or similar in Corona.
If, however, there is a map that can be converted but is not supported by the Converter I may simply have forgotten to do so and I'm always open for suggestions :)
[edit] In case you're not using the latest version (v0.22) I'd recommend to update. If you're using an old one, don't change the conversion modes (object selection and material slot selection won't convert maps).