That's an interesting perspective. I agree that "this is a set law" can be perceived as anything not covered being construed as legal (although not necessarily moral). Which could lead to the creation of a new law, or the change in an existing law, to cover the new territory. I also agree that with a guideline a lot depends on acting in good faith. I think the Prime Directive is vague when the situation is the destruction of a civilization due to a natural cause, not from the actions of the civilization.

Say a natural event -- a meteor or asteroid or solar flare or something -- will cause an extinction level event on the planet with a pre-warp civilization. And, Starfleet has the means to prevent such a thing. As long as the pre-warp civilization never knows that their destruction was prevented by Starfleet, whether choosing to save the civilization or to doing nothing and allow it to be destroyed, are both following the Prime Directive. For me, that's where the Prime Directive is vague. Or, quite likely, I'm unsuccessfully trying to process two vastly different outcomes having the same value, i.e. not violating the Prime Directive.