Are salary employees generally allowed to be paid for their vacation time in lieu of taking the vacation, or is that included in their salary? Our hourly employees are allowed to receive pay in lieu of vacation time.

Depends upon the company rules, should be in a HR handbook. Company I worked for allowed for rollover time limited to one week, all time over that was lost.

If you leave the company with vacation time accrued, you generally are compensated for the time.