The negotiations on the fiscal framework have become a game of chess - both sides would prefer a deal, but both are trying to manoeuvre themselves into the most favourable position if a deal isn't possible. The London Tories want to be able to say -
"We tried everything possible to deliver The Vow, but the SNP sabotaged the whole process with ridiculous demands."
While the Scottish Government want to be able to say -
"The Vow has been betrayed. Nobody told No voters they would only get the new powers they were promised in return for a £3 billion cut in Scotland's budget."
As you can see, either way The Vow won't have been delivered, so this game is far more dangerous for Cameron than it is for Sturgeon. At best, he can find a plausible excuse for his own failure to keep his side of the most important bargain London has ever struck with the people of Scotland. At worst, the charge of a deliberate betrayal of The Vow will stick, thus bringing about one of the two obvious casus belli for a second referendum that we always knew were possible (the other being Brexit).
At the moment, the game seems to be moving decisively in the Scottish Government's direction. With six out of the ten members of the Smith Commission (including representatives of four of the five parties involved) saying they broadly agree with the Scottish Government's interpretation of the 'no detriment' principle that was at the heart of the Smith package, it's going to be almost impossible for Cameron to establish a narrative of The Vow being sabotaged by the wicked SNP. He now has two choices - make a more credible compromise offer, or accept the consequences of his government's double-dealing.
For my own part, I actually take a different view from the majority of independence supporters on social media - I very much want a deal to be struck, even if it involves some compromise. I want the Scotland Bill to come fully into effect, so that we have something tangible to show for the success of the Yes campaign. But even I accept that we can't take that at any price, and at the moment the price looks far, far, far too high.