I agree with you that it seems like a good idea. However, the vacancy tax is still in its infancy, so we've yet to fully understand its effectiveness or side-effects. A big issue around the housing discussion in Vancouver is the lack of data and amount of misinformation being injected into the discussion.
The Netherlands has a system where vacant properties may be occupied by squatters after they've been empty for a year. There's an active squatting scene in Amsterdam, particularly OT301 (an abandoned cinema, now an arts centre).
Enforcement? People gaming the system so it doesn’t look unoccupied?