Mushrooms are not vegan material. At least not if they want to be 'pure' as their philosophy demands.
Basically if you eat mushrooms you are indirectly eating meat.
I'm reminded of the Far Side Cartoon where everyone decides to become a vegetarian, and through a reducto ad absurdem argument such as you are offering, end up hanging upside-down on tree branches because they don't want to step on bugs, either. The vegetarian rule is simple: don't eat anything which has eyes, parents, or screams when you kill it. I've heard a hundred variations on the silly "plants live off dead matter" argument; you're hardly original.