Cash flow is the money you have left AFTER paying all expenses, which includes setting aside funds for future maintenance, capex (roof, carpet, furnace, driveway, and other big expenses), vacancies, property management, etc.

Should I be weary of this, or just go for the cashflow opportunity and factor in doubled vacancy rate on the OPEX? 

Me personally, I would factor in property management and vacancies.

Are you having any issues with vacancies or moving properties if need be?

Long term tenants mean less vacancy so maybe over time lost rents are somewhat mitigated by fewer vacancy expenses... or maybe that's naive.

Vacancy for months? 

The property manager stated section 8 doesn't have high vacancies and that rents are direct deposited. 

It is a desirable property in a growing area, and I've run a stress test - assuming I have to drop rent, have 8% expense ratio and 15% vacancy rate, I'm not dropping to the negative, but have a rather weak 2% ROI.

Now you have a fair market offer (purchase price+repairs = ARV).From there, I like 10% CAPEX, 10% maintenance, at least one month vacancy (sounds like $3100 a month?)

Haven't had a vacancy in 4 years.