From: Mike .
I've seen vacancy rates published for cities as a whole, but not individual suburbs. The only way to determine the vacancy rate for a particular suburb is to ask the RE agents covering the suburb for the size of their rent roll and the number of available properties to let. Those two figures give you the vacancy rate. Average it out over 3 or 4 RE agencies should give you a fairly good picture.
Another way to get a "feel" is to ask the RE agents when they last raised rents and will there be a rent rise in the near future. If there is a long period between rent rises then their is either an oversupply of properties or little rental demand in that area.