Can a vampire use blood to ghoul a carnivorous plant, like a Venus fly trap?
Nope, according to the rules (click HERE to see the White Wolf Wiki page for the Ghouls that clarify your question).
Mortals (a "normal human being" in the classic World of Darkness, see HERE for further informations) can be ghouled, and also animals, but they tend to mutate for the presence of Vitae in their system.
Citing the Wiki:
Ghoul is a term used to describe a minion created when a vampire gives a bit of his or her vampiric vitae to a mortal without draining his or her of blood first (which would create another vampire instead).
Animals can also be ghouled, a tactic particularly favored by the Nosferatu. Animal ghouls tend to grow larger and mutate, the result being a somewhat horrifying parody of the original creature. In addition, almost all ghouled animals develop a taste for blood, regardless of their original diets.
A vampire can temporarily control plants using the Thaumaturgy Path "The Green Path".