A more likely explanation is trade and cultural exchange [EDIT] and expropriation. The Vikings roamed far and wide [0] [1], and Islam even made it as far as Spain at one stage [2]
As far as they went though they probably never got as far as East Asia where they could have encountered Buddhism.
[0] https://en.wikipedia.org/wiki/Vikings#Viking_Age
[1] https://en.wikipedia.org/wiki/Vikings#/media/File:Vikings_ex...
I've read that Vikings were kind of half traders, half raiders. It was a culture of 'bringing back the spoils', I think the word was leader was actually something equivalent to 'ring-giver', and the bond of allegiance was made on that basis. So I suppose you could accomplish that goal in a variety of ways, depending on the circumstances, size of the group, access to weapons, or information, and so on.
Also worth remembering the "the vikings" where not some homogeneous society. The term covers a large number of different groups from different countries and different periods of history. Some traded, some looted, some went west, some went east etc.