I'm not an expert on Parliamentary procedure but I think what happens is that a vote only happens if someone formally objects to the motion to carry the Bill. Given that this involves a Division of the House, and that there was no doubt that the Government would have carried the motion, then given the sheer amount of business (I am amazed the debate actually went on for as long as it did - some five hours) I suspect that the LibDems decided it was not worth even making a show of opposing it. I look forward to hearing more analysis.