Virtual domains must be accessed using the specified virtual domain name, since that is how SME picks which website to return.
To access a virtual domain from the Internet you must configure the new domain on a public DNS server, and configure that server to point to your SME's IP address. (see
http://wiki.contribs.org/SME_Server:Documentation:Administration_Manual:Appendix#Appendix_B._DNS for some general DNS info)
For testing, you can create a 'hosts' entry for the virtual domain on a workstation.
In windows the 'hosts' file is located at
c:\windows\system32\drivers\etc\hosts. You can add an entry to this file using:
echo a.b.c.d mydomain.com >> c:\windows\system32\drivers\etc\hosts
or:
notepad c:\windows\system32\drivers\etc\hosts
Be careful in the first case to use the double >> so you don't overwrite your existing hosts file. Be careful in the second case that notepad doesn't automatically add ".txt" to the file when you're done - 'hosts' needs to be just that - 'hosts' (not 'hosts.txt').
If you get a 'hosts' entry created successfully you should be able to
ping my.virtualdomain.com and have it display the correct IP address in the output (whether or not the ping succeeds). Note that
nslookup my.virtualdomain.com will fail until you get the public DNS server setup as nslookup does not examine the hosts file.