Medievalists.Net brings us a golden oldie from the Proceedings of the Sixth Viking Congress, 1969, entitled Vital Factors in the Success of the Vikings by Bertil Almgren. The thesis seems to be that the success of the Vikings was due to the shallow draft of their ships, which allowed them to travel farther up river inlets and over a wider range than other contemporary vessels. While that's certainly true as far as it goes, I'm not sure that would be one vital factor I would choose.
Bonus; you can download the entire Proceedings book from Scribd at the link above. Lots of good stuff in there by some names that should sound familiar, including Gabriel Turville-Petre.