You seem pretty much there. Are you allowing carryover of vacation time from year to year?

I would use TODAY() instead of NOW() (NOW() includes a time which you don't need)

Once I have an answer to my question, we can tighten up your sheet. I might suggest another sheet in the workbook containing a running total with each line/row simply being employee name, date of vacation start, date of vacation end, total days. Then you can pull "Hours Used" from that sheet and not worry about math.