Looking to understand housing market dynamics

Rental Vacancy Rate is a high value data point for the work that I do.  

I've noticed that table DP04 calculates a Rental Vacancy Rate.   It's not entirely clear to me how this number is being calculated.   When I've calcualated the rate manually based on values from S2503, I come up with a higher rate.  

Is there a way to know the algorithm and what all of the inputs are for a given table like DP04 in this case?