How would wakanda's monopoly on vibranium really be like in the real world of macroeconomics?

The rarity of the metal would make it unimportant in mass production.

For example: it would not be important in military applications if one could not get enough to mass-produce whatever weapon needed it...unless you could make a single weapon that was somehow a "game-changer" that made your nation the indisputable sole military superpower.

However: it might still be very important in science.

SO: in general, it's severe rarity would reduce demand for it.

Consider oil as an example. The oil nations (OPEC) can restrict oil export and so increase demand for oil (and price). However: this only works because the use of oil is prevalent throughout society. That is: everyone wants oil products because oil products are widely available. If only one nation had oil and they never - or almost never - exported it, then only that nation would have a huge, popular demand for oil and oil products. Oil products would never have been mass-produced and never become popular in other nations simply because of the lack of availability of the resource.

So: the only way to increase international demand for a product unique to your nation is to export it and, thereby, allow a reliance on that resource to develop in other nations. Once that reliance is established then you can "squeeze" these other nations by limiting the export of the resource (but of course you have to be concerned that some nations may attempt to take the resource by force).

China in the case of the rare earth metal, or Japan in the case of high tech chips.