The reason to use the vacation instead of taking the payout is you get slightly more money that way. This is especially true if you have a month accrued. Consider the following two possibilities:
Option 1: Give notice on Monday, August 23rd. Work two more weeks. Your last day is Friday, September 3rd. You have 20 days unused vacation at that point, which you take as a lump sum. Your employer health insurance will extend through the end of September.
Option 2: Start a four-week vacation on Monday, August 23rd. Give notice the day you get back (Monday, September 20th). Work two more weeks. Your last day of work is Friday, October 1st. September 6th was a holiday, so you only used 19 of your vacation days. You get the last vacation day paid out when you leave, plus whatever you accrued during that extra month on payroll, plus you get your health insurance covered through October as well.
You work the same number of days in both options. You get more freedom a little bit sooner in Option 1, and you get a little bit more money in Option 2. Not a huge difference either way. Choose what you want to optimize for and do that.