Isn't it that adamantium is just really tough and unbreakable while vibranium has other unique qualities like being really shockabsorbing. As far as I can remember anyway.
They might just be used interchangeable though, as just the marvel-way of saying 'really strong metal'. With there being an emphesasis on vibranium some times because it has this really cool backstory with it coming from Wakanda and such. Whereas adamantium just, uh.. appears?
Considering that the definition of adamantium is basically that it is the strongest metal there is, you could just take the current strongest metal and name it adamantium. Vibranium would be cool though. But overall I'm happy with the metals we have, they do just fine.