[Sim9]

Sounds like you cleared it up, but just so everyone knows the distinction in case all they really need is the stock vB functionality:

Stock vBulletin provides an option to control the timeout between two individual PMs (adwade's post) - this lets you say a normal user can PM no more than once per sixty seconds. With a sixty second delay, a user can potentially send 1,440 privates messages a day (1,440 minutes in a day). Setting this to an hour interval mitigates this down to 24 private messages a day, at the cost of potentially annoying legitimate users who now have to wait a full hour between sending PMs.

This plugin lets you restrict the number of PMs sent over a span of a full day (or whatever other interval you set). So you could set the vBulletin delay between PMs to a negligible five seconds, but only allow five PMs per day for new users. This shouldn't affect legitimate new users, but will significantly limit the damage a spam bot could do.

Hope that makes the plugin clearer