Can IO be fired while on approved vacation and then not paid for my vacation?

1) If the vacation were approved, you should not be fired for using or taking the vacation; being terminated for taking something you were told you could do should give you grounds for a lawsuit on one or more basis, such as breach of contract (the oral agreement between you and your employer, by which you work in exchange for certain benefits, like vacation time, as well as pay) or promissory estoppel (the employer promised you that you could take the vacation, you made your plans based on that promise, then they renege on the promise, causing you harm).

However, you could be fired for a reason unrelated to the approved vacation, even while you were on vacation, such as: performance-related issues; violations of company policies; theft or other criminal activity at work; downsizing or restructuring--or if  you did not have an employment contact, for essentially any reason at all that is not illegal discrimination (e.g. not discrimination on the basis of race, religion, age over 40, sex, or disability).

2) Whether you should be paid for any vacation days from that vacation that you did not use yet (for example, say you were going to take 5 days of vaction but were fired on day 2--the issue is whether you should be paid for the other 3 days) or for other accrued by unused vacation days you had depends on your employer's policies.

Arizona does not require employers to pay employes for unused vacation on termination; however, if the employer's policy has been to pay such days, then it must continue doing so until it provides notice of a change in policy. So if people who were fired before you were paid for their vacation days, or if there was some written or formal policy saying you'd be paid for your vacation days on termination (e.g. in an employee handbook), then you should be paid; but if your employer has not made it a policy or practice to pay out vacation on termination, it does not need to pay it now.