The language is not vague, the interpretation is. Comic book characters are connected in often random and unwieldy ways that what defines a "family" can be different to different people. There's a lot of gray area as to what characters are covered, just that most of those gray area characters aren't popular enough to warrant any theme park stuff.
To expand on this, let's look at the relevant passage again: "any member of THE FANTASTIC FOUR, THE AVENGERS or villains associated with a hero being used". Does the clause "associated with a hero being used" refer only to "villains" or to all of the preceeding nouns? e.g. is it "any member of The Avengers" or "any member of The Avengers associated with the hero being used"? What defines "The Avengers"? Would an offshoot group like New Avengers or Dark Avengers count? What defines a character as a "member of the Avengers"? Current or active membership in the comics? What if they were just for a few issues of panels? What if the character was kicked out -- are they still a member? Is SHIELD something that could be used at WDW?
I'm just saying that there's wiggle room for Disney (or their lawyers really) to debate on some characters if they were worth it. But the characters that fall into that category are largely not worth fighting over. GotG, however, might fit.
All this assumes that Disney cares about bringing Marvel characters to WDW, which doesn't seem to be the case considering the (lack of) usage of such characters in other parks. So, it's probably just a pointless debate.