Ok got it - I guess I am getting tripped up because I am working 5 days a week still, just less hours per day, usually 5-6 hours. You are working less days, but a day is still a whole 8-hour day.
Is your vacation calculated in days or hours? Back when I decreased hours with an employer, it was all calculated in hours. If I got 3 weeks of vacation time in a year, that was actually 15 days or 120 hours. When I dropped to 80% FTE, I got 80% of that, or 96 hours. (It was actually a set # of hours per pay period, but that what it was over the course of a year.) As others said, I felt like I came out ahead because things that might have caused me to take a day off before could be done on my non-working day. And things that might have caused me to take a half-day (4 hours when FTE) often didn't require me to take any time. I see that you still work 5 days/week. Is that flexible at all? If you want to take a Friday off, can you just work a couple extra hours on the other work days so you have the free day without taking vacation?
My 403b match stayed the same as a % of salary, but of course because my salary was lower, so was the match. (I actually moved to a different role entirely, which payed more per hour, but I worked fewer hours, so the match ended up being about the same, but that was only due to the raise.)