I don't think doom metal influence is a defining trait of viking metal. It may be that they both draw from a common source of influence (mid-paced/slower heavy metal) but that's simply a matter of common influence rather than of direct influence from doom metal. I would still make a distinction between viking metal proper and viking/doom. The reason that you see quite a few viking/doom bands is partly that the range of tempos that viking bands utilize is ripe for incorporating doom elements (there are further reasons why there are specifically viking/doom bands as opposed to, say, viking/sludge bands, but let's ignore that for the moment.) But I hear no discernible doom influence in, say, Moonsorrow (they're viking metal, right?) What I hear is Hammerheart influence which, by implication, means Manowar influence. I don't hear any non-trivial similarity to Candlemass, for instance.
Actually, I've been listening to Hail to England and Hammerheart a lot lately. It's probably not a coincidence that both of those albums have an attraction for me right now; the similarities are pretty obvious. To call viking-era Bathory a Manowar clone seems like a bit of stretch to me though (and anyway, what are the necessary and sufficient conditions for music X to be a clone of music Y? I do not know.) And even if it were a clone, I wouldn't call it bad. I find most of the songwriting on Hammerheart to be both exquisite and moving.