A bit hard to say. Since there’s two major types of contractors

1. the “basically employees” contractors are ones who do everything expected of an employee but aren’t granted the status of an employee. This is a “simple” matter in enforcing that they be actual employees under the law, either through courts or with a definition of duties that keep them from skirting that line (though employers will always try). Basically make the definition relatively lax so they can’t do stuff like “work 38 hours a week” to avoid it.

2. The “truly contractors” who essentially set their own hours and get paid for work provided. Basically stuff in the gig economy . These are pay as you go so I’m not sure you can enforce vacation time into this model.