Oil leaking from the bottom of the alternator

Would someone please educate me on what the vacuum that is part of the alternator on subject truck? There is oil leaking from the bottom of the alternator which I have to assume is supplied (the oil) by the incoming line to the top of the vacuum. I think I read a thread on the web that said there is a hole on the bottom of the alternator for oil to drain in the event that a seal between the vacuum assembly/body and the alternator were to leak which would protect the alternator. I will note that I found the oil leaking from the alternator when my batteries were being pulled down so I am thinking the alternator is bad regardless of the purpose for the drain holes. If it is a seal and the seal is replaced what are the chances of the alternator still being good given the fact that it is now not replenishing the charge back to the batteries? And please tell me what this vacuum thing is all about and what the purpose of oil to it is for?