"vhost" stands for "vanity host". It's a domain name you can use when you're connected to a server that offers them for use in IRC. That'll usually be a shell account providers server. Instead of connecting as "omg@AOL.com" you can be "email@example.com". If you think this allows you to muck around and do bad sh*t, think again.
If you Google for "vhost BNC" or "shell account eggdrop" or "vhost irc spoof" or "psyBNC" you'll find lotsa info
. Also note IRC clients and anything related are famous for containing backdoors and other "fun" and will lower your systems security posture. Make sure you verify sources before compiling.