The MRTG plugin is for monitoring your own bandwidth via SNMP talk to the switch that your server is connected to.
My guess here is you want to see what your virtual sites are using. For this, there are two Apache modules (that I know of). The first mod_throttle, appears to be the most complete. The other, mod_bandwidth has a similar set of features but differs in some. Should be findable from modules.apache.org or freshmeat.net
Of course, if you don't have root access to Apache and some knowledge with it you probably won't be able to use these.