Cap Rate Question

[Account Closed]

How to properly figure out cap rate on an owner finance deal? All my houses are owner finance.  It's pretty simple to figure out so I thought....

My first property I bought, I figured it like this: 

purchase price: $51,000

rehab: none

taxes: 1215 annually

ins: paid by owner finance buyer

total revenue per year: $9672

So total revenue after taxes is $8457. 

8452/51000 = 16.6%. Seems simple. 

But then someone was asking me about other expenses. I don't have any....but, what about if the property ever goes vacant? Say in 4 years, she defaults? Then I have to spend 500-1000 dollars to foreclose, and it may sit empty for a month or two until I get a new buyer. 

Also, it DID take me two months to find a buyer. So it was vacant for that period.

I don't know how to figure those into it. Vacancy rate is part of the rental equation but I wasn't sure on these deals I do. Thanks for any feedback.