My virtual domain setup isn't working. How can I debug it?

You can use an exim command with -d to get it to show you how it is processing addresses. You don't actually need to send a message; use the -bt option like this:

exim -d -bt localpart@virtualhost

This will show you which routers it is using. If the problem appears to be with the expansion of an option setting, you can use the debug_print option on a router to get Exim to output the expanded string values as it goes along.