Also, knowing Charlie, he probably does his best NOT to think about what she could possibly be and an explanation for her drastic change--he'd much rather stay in the dark than deal with the truth of what Bella and the Cullens are...
When Jake goes to say the word werewolf, Charlie cuts him off and says "unghh" haha I don't know if that's his exact wording, but you get the picture
I like to think that Charlie never really figures out that Bella is a vamp because that way he's never in danger with the Volturi--at least that's one of them
"Amazing", he said, his exquisite voice full of wonder, slightly amused.
"Carlisle was right."