A few years ago I had a job in which I received four weeks of vacation annually. Unfortunately, that position was defunded. When I took my next position, the new employer would only give me two weeks of vacation; when I said I'd rather have at least one more week of vacation they gave me a larger salary instead of more vacation, and then said they were not permitted to match my previous vacation level. Since then, I've changed jobs one more time; I was able to get a third week of vacation, but they also preferred to pay me more, rather than to give me a fourth week of vacation.
The industry I am in usually works by contracting services to other organizations. I realize that my employer doesn't get paid when I take time off, and that vacation actually costs them money. However, with the family responsibilities I have, I find that three weeks of annual vacation is insufficient for me to meet family obligations and also take a "real vacation" which allows me to properly re-charge.
When I pursue my next job, what arguments and negotiation tactics can I use to get more vacation time?