For OSJ - IMHO, it probably wouldn't hurt your application, but I don't know how much it would help. I noticed that their other essays were shorter than the VMCAS PS so unless it contains a lot of pertinent info, they may not really read it and it may not really help your app either. If they wanted a whole 600 word PS they would have asked for it. Maybe you could condense it and still get the info you wanted across to them.
For VAGirl - I thought the Tufts app was more user friendly than VMCAS. It is a bit more time consuming because they have a couple of shorter essays instead of one bigger one. Also, they have more specific requirements for the people who write letters of recommendation for you and, yes, the letter are separate. To me its completely worth it because Tufts is one of my top choices. It certainly is less time consuming than Cornell's supplemental even though they participate in VMCAS.
My own question - Did anyone else run out of room in the animal experiences section? I was thinking of putting my additional experiences in the additional info section. Does this seem reasonable?