One of my former coworkers went at least a decade without taking a single vacation day. He cashed them in every year. Eventually one year him and his wife went on a vacation and I guess he took a liking to the experience because from that point on he always used up all of his vacation time. All it took was him actually experiencing an actual vacation trip to see the value in them. It really annoyed our manager, from whose budget vacation payouts didn't come, when he no longer had one of his most productive engineers in the office all 52 weeks of the way, especially since the coworker had been with the company long enough to get six weeks per year. Though not completely due to the newly discovered fondness of using vacation days, we did end up increasing headcount around that time and I'm sure it was a contributing factor.