The oldest record of Ashkenazi minhagim extant is Machzor Vitry,which was compiled by Simcha ben Shmuel, apparently of Vitry-sur-Seine southeast of Paris, who was a talmid of Rashi. The Rosh says that Ashkenazi minhagim go back to the Bayit Sheini. This is probable as the Jews in the Rhineland sent their questions to the Geonim of Eretz Yisrael whereas the Jews of Spain sent theirs to the Geonim of Bavel.
BTW, the question of Sephardic minhagim today was a major disagreement between Rav Ovadia on the one hand and Rav Mordechai Eliahu and Rav Shalom Messas on the other hand. The latter insisted that the different Sephardic communities should keep their own minhagim and, in fact, Rav Eliahu paskened according to the Ben Ish Hai (he was, in fact, a descendant of the BIH’s sister).