Unfortunately, some of the current reasons you'll find vacant homes is because homeowners are possibly unable to continue paying their mortgage payment and are in the process of losing that home. So they're not likely making two mortgage payments.
On the other hand, there might be some who have had to move from their home due to a job/locaton change, and have not yet put their home on the market.
I believe it's safe to say that there can be many reasons why you'll see some empty homes. So I wouldn't make any assumptions as to what their circumstances are for leaving the home empty.
As far as you getting ready to sell, it is probably a really good idea not to buy another property until you've sold yours. Here are a couple of reasons not to - 1) As you mentioned, you won't have to be making 2 mortgage payments; 2) The longer it takes to sell your home, the longer you would be making 2 monthly payments (not counting 2 sets of utilities, homeowners' insurance, and taxes), if you chose to purchase another one beforehand; 3) You'll definitely have less anxiety, not having 2 mortgage payments at the same time. These are just a few reasons not to purchase another home before you sell your current one.
If you are successful in selling your home promptly (which may be very possible in our current market, depending on the price point and location of your current home), you could very well end up not being able to find the 'right' home for a while. In that situation, many have moved to apartments or condos, or possibly even have lived with relatives.
There are some possible creative solutions, however, in finding your next 'right' home. Please feel free to contact me, and I'll be happy to provide some suggestions in that regard.
Fairplay Realty Elite