Not sure.
I have mostly finished looking through the 'vm ' 42 resource. It looks like it is essentially a wrapper for NK VMCalls. It puts a lot of function pointers in VMVectors, and it initializes its globals, but I am not sure it does a lot of stuff itself. I don't even think that it takes over the bus error trap. It definitely doesn't read and write pages to disk. Somewhere in the vast assortment of Mac OS 9 code there is something which actually uses this API to provide VM services. My best guess for its location would be the Process Manager, Mixed Mode Manager, or the Memory control panel.
MPLibrary has a lot of functions related to the Area VM system. It might do MPTask VM itself, or it might offload it to something else. The 'ager' MPTask might be interesting to look at, because it apparently does page aging stuff.