Vendor Take Back Interest?

[Genevieve Vienneau]

I came across some properties that I'm interested in purchasing. The seller is offering vendor take back mortgage for 20%. I'm trying to run my numbers. What kind of interest rate and terms can I expect on that 20%?  Does it vary greatly or is there a standard? Is this something that I would include in my offer? I have no ides what's reasonable.  I guess a good start would be to check what works with my numbers but I would like to get an idea. Thanks!