A literary critic once said of Dickens' characters that they were not
fully fleshed or three-dimensional, but that they seemed to be because
they "vibrated very quickly" -- or words to that effect.  What is the
exact quote, and what is the source?

I believe you are thinking of this quote by E.M. Forster:

"Dickens' people are nearly all flat (Pip and David Copperfield
attempt roundness, but so diffidently that they seem more like bubbles
than solids). Nearly every one can be summed up in a sentence, and yet
there is this wonderful feeling of human depth. Probably the immense
vitality of Dickens causes his characters to vibrate a little, so that
they borrow his life and appear to lead one of their own. It is a
conjuring trick; at any moment we may look at Mr. Pickwick edgeways
and find him no thicker than a gramophone record. But we never get the
sideway view."

(from "Aspects of the Novel," by E.M. Forster) 

If this is not the quote you seek, please request clarification, and
I'll gladly resume the quest.

Best regards,
pinkfreud

Thank you.  I'm not certain that this is the quotation I had in mind
-- but I'd bet that it is, and even if it isn't, I'd venture that
whatever I have in mind owes something to this passage.