No you will not become a vampire by casting a spell on yourself. There is no way to become something you are not. It would be like saying I want to change my gender. You were born a vampire or you werent.
People may be telling you to use caution when working with black magic. The dark arts are very addictive and alluring to those who have not practiced them before. The dark arts also can come with a price, and it is you that will end up paying that price in the end. The consequences may not be immediate but they will appear soon or later. Prepare yourself and do your research first before practicing with the dark arts.