While not as sharp as a real vampire's teeth, Heidi's charmed fangs were sharp enough to break skin, if she used enough pressure, something she doesn't think she would do unless very tempted or upon request. That doesn't mean she disregaurded the idea, in fact, she did find it tempting, whether due to their game, or her flask, or just Tracey herself.
When the Slytherin's fangs met her shin, they elicted a gasp from Heidi, her knickers almost instantly flooding. "Merlin...."
Her hand found it's was to be woven it to the golden threads of Tracey's hair. When Tracey massaged her brest, Heidi arched into the touch, gasping again.
She hadn't meant to let Tracey take control, and she still planned to fight for it, but at the moment, this aggressive side of Tracey was a very big turn on.