I believe this is responding to a symptom (vacancies) without addressing its primary cause (land speculation.) Housing is viewed as an investment; speculators capture the value of location and public spending by increasing the rent they charge, but don't add any value themselves, essentially siphoning value out of the community. The result is a boom/bust cycle and unaffordable rents. Disincentivizing vacancies may increase supply and lower prices, but occupied properties still have the same fundamental problem. A land tax on the unimproved value of the land is an elegant solution which incentivizes adding value to property rather than using it to park money.

Edit: I'm surprised I'm being downvoted so much - maybe you disagree, but this is at least an attempt to add to the discussion.