The feeling on how common armor was in this era fluctuated and gone through phases when i was a kid it was thought to be really rare. Modern testing has shown just how protective a nice chain mail shirt and a helmet is especially against sword cuts which were one of the really big threats during this time period. The people who formed these retinues of elite troops who were well armored and well equipped and well trained and very experienced with you know elite sort of status These people formed either the tip of the spear for the Viking forces or you could mix them through the formations of the lower quality troops to stiffen them as it would be called.