And what's a "vacant house"?  If a snowbird decides to go to his trailer for the winter, does that make the house vacant, and subject him to tax?  And why should it make any difference whether the house is vacant or occupied anyway?  The City still gets its municipal taxes.  In fact, if the house is vacant, that's a bonus as no one is using municipal services.  So they're still paying tax, but don't add to the user burden.

If the problem is that speculation is increasing the cost and decreasing the availability of affordable housing, isn't that just a part of the capitalist system?  What next, a special tax on market investments, based on their popularity?  Just like housing, increased demand for a particular stock will increase its cost.

If the government wants to subsidize housing, it should do so by spreading the burden over society generally, rather than asking one group (investors) to subsidize another group (tenants and potential home owners).