should 2+ Vulnerabilities give diminishing returns?

[Anaraxes]

Originally Posted by Plane

You can prepare for "use metal against my foe" more easily than "steel or lead" though, since you have extra options.

I'll just pop out to the sporting goods store and get some osmium bullets and a rubidium-technetium alloy hunting knife, just in case. That's easier than steel or lead.

At some point, you're just decomposing one category into relative availability of its components. "Any metal" is in some mathematical sense more available than any two metals, but the customs, technology, and economics of the setting are what really influence how common something is.

I think one simple principle for evaluating a proposed build is that subcomponents can never add up to more than the base category. I'd raise an eyebrow at any multiple Vulnerability just from the point of view of "how does this make sense in the character concept?", and would happily suggest just bumping up the category to cover multiple related banes.

The "prepping for the enemy weakness" is a worthwhile consideration, too. What are the odds of a fragment of another planet winding up on Earth? Not zero; we have Martian meteorites, and Oumuamua shows that even planets from another solar system aren't mathematically impossible. But how much kryptonite actually shows up in Superman stories? The stuff is practically common, in multiple varieties. GURPS Disad values have a strong "do they affect the game" narrative quality ("a disad that doesn't affect the character isn't worth any points"), rather than just a pure simulationist model of a random sampling of the world environment. "Common" means "likely to come up in a story" more than it does "likely to be found in a random kitchen drawer".