I also encountered the vocalising/non-vocalising thing when going into speedreading.
I can supress the vocalising with a bit of conscious effort but it's not the vocalising that limits my reading speed. It's all the rest that goes on in my mind that processes the information I just read. If I'm looking for some specific words, not vocalising is helpful but if I need to remember and understand the text I'm reading, I do vocalising as that doesn't impact the rest that goes on in my mind.
So I'm not a speedreader but it did help me to improve my glancing skills and if I need to I can count really fast now by visualizing the numbers turning into each other.
Seems we go from reading out loud, to reading in our heads (internally-vocalising), but you can continue and read without the internal-vocalisation. I can do it a bit, but under test conditions I only picked up about half of what I would do when I read to myself in my head.
Amazing how different people's headspaces are.