While English allows us to completely elide the thing which is not known, I don't think the Klingon does. It should be, <wej vISov.>
There are some examples that seem to go against each other, and in particular the way in which {yaj} is used ({jIyaj} and {jIyajbe'}; only seldom {vIyaj} and {vIyajbe'}) makes me think perhaps there should be no prefix.
Still, there is General Chang's {DaSovbej'a'?} in Star Trek VI and Klaa's {tugh luSovbej.} in Star Trek V, so I can go for a {vI}- here.
<yaj> seems to be a special case. I wonder if a general understanding is seen to be sufficient to say you "understand," but specific knowledge is needed to claim you, "know." But it's just speculation.
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