California is a great place to work. We have the sun, great weather, and plenty of things to see and do when you're on vacation. Speaking of vacation, California also has laws that protect the vacation time that you accrue at your job.
No law requires employers to give vacation time to their employees. If they do, however, then that vacation is considered to be wages for that employee.
The fact that vacation is a form of wage comes with a lot of implications. For example, once you have vested in your vacation time, it can never be taken away. "Use it or lose it" vacation policies, which take away your vested vacation if you haven't used it by a certain time, are illegal in California. Instead, employers are allowed to cap your accumulated vacation, meaning that, after a certain amount of accumulated time, you don't build up any more vacation until you use some and get below the cap again.
When your employment comes to an end, either voluntarily or involuntarily, you are entitled to the cash value of your unused vacation. That value is determined using your current rate of pay. So even if you worked at a company for 15 years and never took a break, when you leave all of your built up vacation is paid out at your current rate.
Employers can tell their workers when and how much vacation they can use, so as to promote the business effectively. They cannot, however, use their discretion in such a way that it undermines the value of the vacation, such as never approving its use or unreasonably restricting it.
Your employer can require you to take some vacation if it feels that your work quality is suffering and you need a break. It probably can't require you to take vacation for the sole purpose of benefiting the company financially. So if you have a huge bank of vacation and have given your two weeks' notice, your employer probably can't require you to use your vacation for the last two weeks instead of showing up. (This is true for public employers. While the issue has never been decided for private employers, the reasoning is most likely the same.)
Because vacation qualifies as wages, it is subject to all of the laws which protect wages. California jealously guards the wages of its employees, and failure to follow those laws can come with stiff penalties.