Part time employees pretty much don't get any vacation, so I doubt that is the real average (the article says 15, which still probably isn't the real average). BLS says ~16 for full time, so there you go: https://www.bls.gov/news.release/ebs.t05.htm

That's also a hell of a 'so long as'. If you ban lumped PTO and don't establish a robust minimum, lots of people aren't going to get what they would prefer.