I finally got the article that explains broadband coupler design. The coupler is based on the resistive bridge. The coupled port forms one of the arms of the bridge and it is also ground referenced. The DUT port forms another arm of the bridge. Unfortunately, that arm is not ground referenced. To measure a DUT that is ground referenced, a balun is needed to convert the DUT to floating load for the bridge.
This article helped me alot to understand what's going on in the VNA bridge. It doesn't mention the tricks needed to get the bridge working up to several GHz, such as the balancing the parasitic capacitances of the bridge components.
The article is "Simple SMT Bridge Circuit Mimics Ultra-Broadband Coupler" by Joel Dunsmore in RF Design magazine, Nov. 1991, pgs 105-108.
I'll scan the article and put a link to it later.