According to Savills, as of Q4/2013, there were 18,003 vacant units out of the current 289,370 available private homes across the island.
This translates to a vacancy rate of 6.2%, inching up marginally by 0.1 of a percentage point from the previous quarter.

Here's more from Savills:

The increase was due to the spike from 2.7% to 7.1% in the North-East region, whereas vacancy rates declined in the other regions.