propheses are not vague. Since history repeats itself.
And the longer you live the more history repeats itself, so Nostradamus was born 1503 and he started prophesizing in 1547 but published his first opus in 1555 so being at a well matured age (hopefully without delusions), I'm sure he had a good idea of how history repeats itself. I would also guess he was up on his history of the world in segments of centuries too.
If we are to believe history, regions of the world were not in touch with other regions; there were no televisions, telephones, or internet, so news travelled by horse, boat, or carrier pigeon. To be up on the history of the world in segments of centuries would have been highly unlikely for anyone. Personally, i don't believe in this history, but I bet you do.