If you want full time hourly employees (40 hours per week) to earn two weeks of paid vacation per year, then you have hourly workers accrue vacation time at a rate of 1 hour vacation earned for every 25 hours on the clock.
This system handles people who hours worked varies from week to week, and it handles people who have not been there long enough to earn the full two weeks--ever 5 weeks they have accumulated a vacation day, so if they want to take a week off 6 months in, say, that works.
It also works for tracking vacation time for salaried employees. For salaried employees you just force the hours worked to 8 per day in the program that calculates vacation accrual, regardless of the actual hours worked (assuming that you even track actual hours worked for salaried employees).