Different people have different experiences that they can learn from. You need to use the method that works best for yourself. I'm an advocate for vent adjustment. There is a balance between having the vent too closed (which can cause condensation and loss of heat), or to too wide open where you are loosing too much heat through the opening, thus bringing your temperatures down. I rarely run wide open, even when I only had one element. I have never experience such a slow rise in temperature, even during much colder weather. Probably using hot water in a small enclosed cabinet that was at 51°F saturated the air and brought the RH to 100% (or close to it) rather quickly. Also if you replaced the small bowl with a large pan, you are creating a situation that is the same as cooking a load of chicken with skin. If you are using a pan of water, you have a large surface of water, which can put out a lot of steam (evaporation).
Next time take your experiment one step further. When you are running with the vent wide open, and after it gets to your set temperature, start closing the vent and see if the temperature either goes up or down. Of course this will depend on what you have in the Bradley. If it is a load of chicken with skin, then you may want to remain 3/4 to fully open until most of the moisture is expelled.
