This is not possible to do in the same way as it is with Apache / http, this is not a failing of ProFTPD but rather a problem with the basic ftp protocol which as no method of uniquely identifying the target host during a session. The only work around at this time is to use a different primary port for each virtual if more than one per IP is required.
If the <VirtualHost> block is built using names rather than IP's there[A is a chance that a configuration reload will cause the server to die. Proftpd treats DNS resolution as a fatal error "Fatal: unable to determine IP address of `www.blah.com'". The best solutions to this problem are either to use raw IP addresses in the config thus removing all the resolution problems or to use the -t option to check the config prior to reloading.