The rental vacancy rate is what percentage of the time a unit or the the unit (depending on the number of units) is vacant. A terrific occupancy rate is right around 95%. I think there are a lot of neighborhoods that have few vacancies in Portland...including ther downtown area, NW, Pearl, Bellmont, Irvington, anything close to a college, Lake Oswego etc etc. On the flip side though, when you are attracting college students you will tend to get lower rents. These are all things that you should discuss at length with your realtor...and if you aren't currently working with one I would be happy to chat with you. I also have a terrific program on my computer that is an investment property analyzer...basically you plug in all the info about the property including purchase price, taxes, any HOA's, what the rents are, how much you are going to put down, your mortgage rate etc and it tells you the cap rate, gross rent multiplier, cash flow etc...I would be happy to run some scenarios for you if you would like. Otherwise good luck to you, with the rental market at an all time high, interest rates down, lots of inventory and the decline in prices it is such a terrific time to buy!