Wakandan Vibranium (the most well known type, hereafter called vibranium is different from the Antarctic version which we shall call hereafter "anti-metal") was until recently, the most plentiful source of vibranium on Earth. Almost all known vibranium was found in Wakanda and for the most part was the only source whose supply had been regulated and controlled for commercial use. This mountain-sized supply vibranium made Wakanda one of the richest nations on Earth.
Suspected origins of vibranium
Vibranium is an extra-terrestrial "naturally-occurring" metal or so says the Marvel Universe writers. If such a material could appear naturally, it would have to come from the action of supernova where all other heavy (exotic) metals are born. Thus all "natural" deposits of vibranium of either type are from space, most likely arriving on Earth as meteorites. There could be a potential of the material being part of the solar disk as the planets in our solar system formed but it seems less likely given the strange properties of the metal. If it was part of the planet when the Earth was formed, it might have slipped into the core of the planet and only appeared during seismic activity.
Given the exotic nature of the material, being created by advanced being such as Galactus or the Celestials seems a more likely origin, considering nothing else on our periodic table even comes close to having the properties of either version of vibranium. It has been insinuated the Celestials use the material in the creation of their mighty and alien armors. Being manufactured by super-advanced aliens such as the Celestials or Galactus (who are certainly capable of creating such an exotic material) makes much more sense and allows for the material to violate the periodic table with its fantastic properties of vibration dampening or metallic bond disruption.
History of vibranium on Earth
There have been isolated instances of vibranium all over the Earth, appearing in a variety of stories in the Marvel Universe (the fictional South American nation of Tierra del Maiz, was conquered by Diablo for its modest vibranium stores). The development of Captain America's shield is one of the most well known. The material was believed to have arrived on Earth, as part of a meteor shower and would explain why there are isolated chunks of vibranium all over Marvel Earth #616.
Most of the other stories include the name of a company called Roxxon who sought to exploit vibranium's numerous processes for financial gain and control of the planet's markets on the materials. One major storyline revolved around an island in the Atlantic having one of the largest supplies of vibranium outside of Wakanda. That supply was later destroyed by Roxxon.
Roxxon tried to creation a synthetic version of vibranium called Nuform. It was highly unstable and degenerated temporarily into the anti-metal version becoming highly dangerous and having to be destroyed.
In the recent Doomwar Saga, Doctor Doom attacks and attempts to take over Wakanda for its vibranium. In a desperate move, the Black Panther is believed to have destroyed the largest cache of vibranium in Wakanda as well as rendering any easily accessible vibranium derived from Wakanda, inert. What this will ultimately mean to heroes who are using it, depend on it or have it as part of their powers or arsenal is anyone's guess.