how did you arrive at a 5% vacancy factor.

But the biggest factor is price. 

Could be the building, could be your unit, could be a number of factors. 

My biggest concern with refinancing is my credit 640 and the fact that I am now a one income household and had a career change that makes me significantly less money than what I was making when we purchased the duplex ($64,000 per year vs $27,000 although once the rental is factored in it’s around $70,000).

Depends on a lot of factors though.Ultimately it’s best to consult a tax professional about your specific situation. 

Factor the worst case in every investment and you should be on track to make the HELOC work in your favor. 

Afterall Austin since the year 2001, has average 5% appreciation YoY.Do you guys factor in appreciation, or is it like “icing on the cake”?

The tenant makes 65k a year, as that’s a factor.

What factors would go into your decision?

Additionally there's another factor...the bank of mom is going to be loaning (gifting, essentially, as an early portion of our inheritance because more than likely she'll be fortunate enough to leave my brothers and I money when she passes way too soon due to this horrific disease) us money for the down payment on the new house.