This would probably be a problem on networks which unload HostServ's OFF command to prevent ban evasion, though a simple reconnect can fix it, it's easier tracked by the IRC-Ops and more inconvenient to the ban-evader. This is near-impossible for completely innocent users who are trying to /cs unban themself. The only way around this if the OFF command is unloaded, is to group/register a nick without the vHost.
I think ChanServ should probably check for both the vHost AND the real host (at least with certain protocol modules) when using !unban [nick] and /cs unban.
Just my opinion, though.
Edit: Oh wait, didn't see this was a 1.9.x post. Can you unload the HostServ->OFF module in 1.9.x? I sure hope so.