Yes, we are discussing a hypothetical, from a few parents up: "set a property tax that would hurt if the buildings became vacant"
The definition of vacant is something that would have to be figured out, but it's not impossible. For example, you could do a generous 6-12 months of the year occupation without taxation, and then a sliding scale from there. (So you pay 0% of the new tax at 12 months yearly occupation, 0% at 6 months, 50% at 3 months, and 100% at 0 months.)
It seems to me that we could start with a conservative approach and adjust from there. For example, define a property as vacant if it isn't occupied for 1 continuous month or a total of three months out of the year.
Also, you need to figure out how you define vacant, and how to track it.
If I relocated for work, and use the house 3 months a year and every other month for a weekend, is it vacant? How do you tell?