It's possible that his vampire nature could interfere with his magic at times. Maybe the need for blood could disrupt his concentration when performing complex spells. But on the other hand, his vampire strength could give him an edge in physical magical combat, like in the case of certain types of transfiguration that require physical force.
His magic could be enhanced by his vampire abilities. For instance, his speed as a vampire might make his spell - casting quicker.
Well, think about it. Harry's magic is based on his wizarding heritage and training. But as a vampire, he has these new instincts and powers. His magic might adapt to his new form. For example, his Patronus might change in appearance or nature. And when it comes to defensive magic, his vampire reflexes could make him better at blocking spells. At the same time, his need to avoid sunlight due to his vampire state could limit when and where he can practice magic outdoors.