You can get vacancy rates, prevailing rents and other demographics info by running a city report at: http://www.PhoenixBankOwned.com . For all your management needs, contact http://www.AzRentalHomes.com . Feel free to contact firstname.lastname@example.org with specific questions.
Rental data like this is hard to come by on the sub metro level. It is likely you would need to talk to a local pro who can do the manual leg-work to harvest available data from MLS, craigslist, experience etc.
That said you can see some high level housing and rental stats and charts for Phoenix vs. other nearby cities here: http://www.managemyproperty.com/find/arizona/phoenix-propert
Accurate data regarding the Phoenix area rental market is difficult to find. MLS has perhaps only half of the total homes for rent across the valley listed within it, at least until you get to the higher priced rentals. Craigslist has a higher percentage, however it's not a database, so it's difficult to know the status of what leased, for how much, and how long it took. Plus, there is a lot of duplication in craigslist.
All of that said, in my experience as a property manager/leasing agent over the last 5 years, the rental market right now is pretty good. Homes are renting for about their historic norms and when marketed properly are renting in around 30 days. Newer homes are still renting faster than older homes, when an area offers a choice. Condition matters greatly. Move-in ready condition being very, very important.
To get more specific with you, I'd need to know more about what type of investment you are thinking about. Feel free to contact me if you want to chat in more depth.