It should have automatically setup IPs, but just in case it hasn't...
Go into /usr/local/vesta/bin/
These are all the scripts VestaCP uses (from the web interfaces). I'd probably say start by messing around with this (maybe run v-update-sys-ip?) and go from there. I can't really say I experienced that (never had that problem come up before) so I can't really tell you what needs to be fixed.