However, having just had a look at said provision, I cannot see how it can be interpreted so as to permit Vince Cable to abstain on the vote.
"If the response of the Government to Lord Browne's report is one that Liberal
Democrats cannot accept, then arrangements will be made to enable Liberal
Democrat MPs to abstain in any vote."
It's pretty straightforward. If a LibDem cannot accept the Government's response, they can abstain. But, as the formulator of the response, Vince Cable clearly doesn't satisfy the "cannot accept" condition. So what's his justification?
There's another point here, and that's the 'country before party' argument. If Vince Cable is seriously arguing that he will abstain out of solidarity with his party colleagues you have to question his priorities. Surely the country's interests come first.