Regardless, it appears you've changed your opinion here within one reply. You first stated that you had a "very low" opinion of Vick. To have a "low" opinion of someone within a relative setting involving competition is to consider him substandard in relation to his peers; to have a "very low" opinion of someone is to consider him to be among the very worst people engaged within his field of endeavor. To have a "very low" opinion of an NFL player would surely place him among the very worst players in the league.
You have now pronounced his status to be "mediocre." As such, your opinion of him has obviously been reconsidered, and it is no longer "very low"; it is "middling."
You're headed in the right direction.
I was using very low in the colloquial, not in the precise form you use. To tell you the truth, this is the first time I have heard of a specific meaning of "very low" although your take of "mediocre" was my mis-statement.
Very low, as I meant it, means he is not a good quarterback. I haven;t changed my opinion of him at all -- don't let my somewhat imprecise word selection suggest otherwise.