I feel they could simply apply the same tax to non US citizens/permanent residents trying to purchase a home in the Seattle area.
The idea of a vacancy tax seems hard to apply while respecting people privacy.
If the owner is a non citizen who doesn't live in the US, you can use immigration records to know of his presence, and they he/she can show rent documents to prove if the house was occupied. But this looks like it would be hard to enforce.
It's interesting to see how other cities are tackling the issue, Paris bumping up the tax to 60% of the fair market value of rent for instance : http://www.zerohedge.com/news/2017-03-07/vacant-homes-are-gl...
This lost also shows that while Vancouver is high on the list, San Francisco and NYC are higher.