While he doesn't name the Count as a vampire while he resides at this castle, Jonathan does see him crawl like a lizard, experiences the seduction of the "weird sisters," and understands that the count has power over other creatures, such as wolves.  Then, in Chapter 4, Jonathan discovers the coffins with Dracula lying inside one:  "There lay the Count, but looking as if his youth had been half renewed . . . on the lips were gouts of fresh blood, which trickled from the corners of the mouth and ran over the chin and neck. . . He lay like a filthy leech, exhausted with his repletion."  Jonathan, terrified, closes the coffin lid.  Shortly after this, Dracula's body is transported to England, Jonathan escapes, and then of course goes mad.  I would think finding Dracula in the coffin confirms Jonathan's knowledge, even though he does not use the word "vampire."  Only later does the doctor introduce that term.  However, Jonathan does call the Count a "monster."