By design, it should already inherit permissions from the parent area. When importing from VW3, however, a lot of junk permissions get added, because vBulletin created empty forum-permission rows whenever you used the feature to Create New Forum Based on Existing Forum. Unfortunately there is not really a way for VaultWiki to know which permissions are junk and which were actively created, but you should just have to revert those permissions where you don't need them, and you would inherit the parent permissions again.