Not much of a wheel if the spokes aren't solid. I think I get it though.
It is important to note, as has already been mentioned, that the Wheel is more of a metaphor than a de facto model of the world. Its not all-inclusive and will not answer every inquiry, hence why there are other models as well (such as the egg-shaped universe).
So the secret of the Tower is CHIM, and a prominent phrase used with CHIM is I, the tower is I, and a wheel turned sideways is I, thus the Tower is the wheel, so the secret of the wheel is CHIM? And the Thief is trying to obtain CHIM from the wheel?
That's the kind of not blatantly obvious explanation to the question I was kind of hoping for.
Yes, it is about CHIM (the Crowned Tower), take a gander at http://www.imperial-library.info/obscure_text/vehk_teaching.shtml
"--The Tower is an ideal, which, in our world of myth and magic, means that it is so real that it becomes dangerous. It is the existence of the True Self within the Universal Self... The Thief is another metaphorical absolute; in this case, he represents the "taking of the Tower" or, and sometimes more importantly, the "taking" of the Tower's secret.
--Imagine being able to feel with all of your senses the relentless alien terror that is God and your place in it, which is everywhere and therefore nowhere, and realizing that it means the total dissolution of your individuality into boundless being. Imagine that and then still being able to say "I". The "I" is the Tower.
--Stand in its flux and remain whole of mind. Look at it sideways and see the "I". This is the Tower.
--One that knows CHIM observes the Tower without fear. Moreso: he resides within."
The Tower is the ideal, something to be attained, something to be conquered - it is a daunting and threatening fortification. The "I" is the Tower, the "I" is the thing that must be reckoned with or conquered or stolen in order to attain CHIM. When you achieve CHIM that fortification is no longer so daunting, you've stolen its secret and "reside within" - in siege terms you've "taken the Tower".
Note that Vivec is the Theif, and that Vivec achieved CHIM.