There was an article saying something like "if a vacation is interrupted, it no longer counts", and it backed that claim with studies. As I tend to agree with it more and more, I want to find that article (or those studies), but I can't. I remember it being a reputable source.
In my interpretation, the logic was this:
While the importance of cutting off during your personal time may be obvious, the studies have proved not doing that may negatively impact the result of your vacation. Merely answering an e-mail (or being forced to think about work in any way) during your personal time may nullify the accumulated rest and reset the vacation-stopwatch back to zero, so that you have to start over.
It was probably phrased in a less dramatic way, but the main point remains.
Could it have been my wishful thinking?