Psydonym,
I have pring for a while now and I still subvocalize sometimes. I do it less when, during rapid reading, when I'm already familiar with the material and more so when I'm not. Personally I wouldn't worry about it. Just follow the steps and it will come. By the way my rapid reading speed varies from 3000-550. But it tend to mix in super reading and dipping with rapid reading. More of a postview like thing I suppose.

Fritz