Hi there! I see you have the same problem I do. While this can be annoying, I did find a solution that worked temporarily without the use of mods or console commands. Now, whether it is through luck or whatever, here is what I did. I took a two handed weapon (the ebony blade, though I doubt it makes a difference) and just kept power attacking vilkas. No one else attacked me and Vilkas kept being staggered, but he eventually said the next line about his sword. Now, he didn't actually give me the sword until I power attacked him a few more times. So give that a whirl and see if it works for you.