J’ai rencontré les végétariens

means “I met the vegetarians”.

J’ai lu un livre de des végétariens

is not grammatical. You might say:

J’ai lu un livre de végétariens

but it is ambiguous, it might mean as well “I read a book about vegetarians.”
As you suspected:

J’ai lu un livre des végétariens

is also ambiguous and can mean “a book belonging to…”

To make clear the book was written by some vegetarians, you might say:

J’ai lu un livre écrit par des végétariens.