Well, you have two big kinds of vhosts:
- setted via a bouncer
- setted via irc server. Bouncer
It's a gateway owned by someone wich allow you to connect to your irc server using its vhost .
Usually, a shell provider could furnish bouncer and howto IRC Server
Some services contains a hostserv (epona, anope, neostats, ...) wich allow you to use a +x flag.
A v-host is actualy just a different name for an alias.
Aliases, just like with peoples names, are not the machines true name, but can be used to refer to it.
They work by setting both Forward and reverse DNS details for 1 specific IP and a sub-domain.
It is possible to do this with dynamic IPs, however, in 99.9999% of cases you can't.
Dynamic IPs are dynamic due to the fact that a ISP can't provide enough IPs for one per customer. Thus they have a pool smaller than there userbase which are randomly shared out. Remember, not all users will be connected to the ISP at the same time.
In most cases, your ISP controls the Reverse DNS information for the IP you are assigned, thus you can't change it. You would need to have authorisation to have a static IP and have rDNS control.
Provided you have access to a domain, and your registrar provide good DNS managment, you can control the forward DNS address without issue.