to my understanding, most of the HR will allow you to take vacation before you actually leave the company, BUT, you haev to show up on your last day of work. i.e. if you have 23 days vacation, and your last day of work (per resignation letter is aug 31), then you could only take 23 vacation day, and come back to the office on aug 31 to sign off all the paper... and take 1 day cash in lieu of vacation.
that's what my company is doing. but for sure, you could not just used up your vacation as the notification....