From the links:
The homeowner vacancy rate is the proportion of the homeowner inventory that is vacant for sale.
The rental vacancy rate is the proportion of the rental inventory that is vacant for rent.
So essentially this is a measure of how many properties are vacant and available for rent or purchase.
So with a population of 340m, 15m vacant properties, and an average household size of 2.5 we have 15,000,000/(340,000,000/2.5)= 0.11, or a possible vacancy rate of 11%. That seems like a lot.
Sometimes people refer to underutilised properties rather than vacant properties for this reason.
I am very interested to know what happens to the proportion of underutilised properties during an accommodation crisis. The most recent linked chart in this thread is very interesting - what caused the steep increase leading up to the GFC (i.e. during a boom - often associated with a 'shortage')?