To be joined at the hip means to be closely connected, and in this context it is used to describe how hard it was to spend a week with Vanessa when he was used to being alone.
Consider the statement "Over the past month I had got used to being alone, and spending a week attached and inseparable to Vanessa working and sleeping with hardly a minute's break, had been hard.". Based on this "Over the past month I had got used to being alone, and spending a week joined at the hip to vanessa working and sleeping with hardly a minute's break, had been hard." a true statement? Yes, no, or I don't know?