The absolute worst-case scenarios on rents would be $1700/mo and even at 25% vacancy it would still pencil out at 9% cap rate.  

The market has improved over the past 12 months and we have been able to push rents 3 times already with no drop in vacancy. 

Is it worth the vacancy and headache?

After taking 7.5 vacancy, 8 repair, and 8 percent capX.  

Imagine the following scenario:Suppose you bought a SFR at a good price (20% below retail) and made very conservative estimates for water, garbage, vacancy, property management, capex, and repairs. 

Vacancy 8%, repairs 5%, capex 10%.

I use 8% vacancy, 5% repairs and 10% capex.

I am more conservative than you, 5% vacancy is 1 month vacant out of 20.

What you can do with more units is mitigate that risk and structure the deal such that even if you have a vacancy in a 4plex, the cost of operating that property is still covered the other 3 units - not possible in a duplex in Anchorage that I've seen.I am also active duty, but have a background in real estate, and my wife is currently a realtor who specializes working with military clients.

You should be running a significantly higher than residential real estate vacancy rate.