Is there anything Vince cable could do that would get him fired by David Cameron?

Declare his own bias in an extrajudicial role - No

Oppose agreed coalition policy - No

Declare that "Conservative governments have dominated ever since without securing a majority of the popular vote... We need to make sure the progressive majority wins elections in this century and not the Conservatives as they did, by the back door, for two-thirds of the last century."
This is not just a pro-AV call it is anti-Conservative one.

In late 2010 Vince Cable declared that he had "a nuclear option... if they push me too far then I can walk out and bring the government down and they know that". Is he right? Can he say and do anything, knowing that David Cameron will have to grin and bear it? Is David Cameron really that spineless?

I know John Major and others have subscribed to the view that it is better to have some opponents in the camp pissing out than outside pissing in but hasn't Vince Cable gone too far?