In my experience, the key is cutting them small before dehydrating, then fully dehydrating them so that they shatter when ground (like coffee beans). I have ground dried veg to near-powder in coffee grinders (reserved for spices), a small mini-food processor, and my larger food processor.
My favorite use for these is to make my own bullion powder: mix 1/2 cup powdered veg with 2 -4 Tbs arrowroot powder, 1
Tsp saltt, and several teaspoons dried herbs (parsley, dill, etc). Mix thoroughly and then mix in 2 Tbs natural oil (sesame, olive). Store in fridge for up to 6 months.
The oil coating keeps air off the veg so that flavor and nutrition are retained. This makes a GREAT soup base!