There are less repair issues than with an SFR, more amenities that tenants like (and yes, the maintenance fees pay for that), and I feel there is much more security during vacancies than in an SFR. 

We buy class A properties because our tenants allow us to keep our vacancy rates, maintenance low.

That results in a very close inspection of the property since its empty when showing and inspecting and its easier to see problems.All that said "vacancy" during turnovers is inevitable in this business. 

However, vacancy has been extremely low and no major repairs so far.

Expenses, vacancy and capital are going to be a LOT more than $20,000 a year. 

So, if your NNN investment is located on a highly trafficked retail corridor, with low unemployment, growing demographics, and low crime, there are likely to be many potential replacement tenants willing to lease up the vacancy.

You want the property for cashflow and you can get started with that right away. the neighborhood won't have to live with a vacancy.

In my areas with a good faith effort, I can get it reoriented with no vacancy.

Vacancy rate would be around 5% or 6%.  

Does the cash flow account for maintenance, reserve fund, & vacancy?