I mean, all the psych research says that it takes at least 4+ days until people really get work off their mind, and the work-mindset starts a few days before returning. So vacations start to be effective at a minimum of 2 consecutive, uninterrupted weeks (coincidentally a legal requirement in a bunch of countries). If you're taking vacation in small blocks or continue to think about work on vacation, then you and the company lose the working days spent, but aren't getting the expected recovery/productivity/burnout-prevention/etc benefits for which vacations are actually implemented.
Classifying any of these as "employee is out taking vacation time" is a giant spin.
That's not vacation, that's telecommuting.