How do I remove the distinct protein flavor of vital wheat gluten when making faux meat?
One way to avoid the vital wheat gluten taste is to develop wheat gluten directly from flour (a somewhat popular food in China). To do so, make a dough of bread flour and water, knead it well, rest 2-3 hours, then "wash" the dough in water until most of the starch has been rinsed away. The result is a high protein/water mass that I assume could be used in faux meats, that will taste highly umami (from wheat protein's glutamate), but not unpleasant like vital wheat gluten.
To wash the dough, submerge it in a bowl of water in the sink and rub it (like handwashing clothing). Change out the water until it turns clean. This process can take ~15-30 minutes and 10+ changes. It's a hassle, but the result, if done well, will have an amazing chew and flavor.
This method has a low yield. One method to increase it involves adding a small percentage of vital wheat gluten to the flour. This increases yield without really effecting flavor, though adding too much can make the dough unpleasantly tougher. Another method to increase yield may be to add salt.
I doubt you can remove the flavor of the vital wheat gluten (most commonly found in the form of seitan) itself, which some describe as tasting more like meat than other meat substitutes, probably because of the protein flavor you suggest. Perhaps a better approach is to use other flavoring ingredients (onions, garlic, spices...sauces...broths...) to mask it's flavor.
I have had great success by adding balsamic vinegar. I also discovered smoking a cooked loaf at 200F for an hour also hides/removes the off flavor. To keep the loaf moist during the smoke, increase your liquid to vital wheat gluten ratio, smoke at a low temp and place a container of broth/water on the rack below the loaf. Cheers
Many use apple cider vinegar (around 1 tablespoon) in with the with wet mix when forming the dough. I've also used sherry.
When mixed it's then best to let the dough rest to allow the gluten to develop but also allow the apple cider vinegar do its job. (It won't taste of vinegar when its cooked.)