to get into UCD vet, you most certainly do not have to attend UCD undergrad. it won't help, and it won't hurt you, to be at UCD for undergrad.
that being said, AP classes -- depends. plenty of people have only taken one or two, whereas someone like me has taken 8 (11, technically speaking, as i entered UCD with college credit).
GPA? aim for a high GPA. davis does admissions by a point system (at least, last time i heard), and it's based primarily on your GPA and SAT scores.