Everyones verson of vampires vary and the rules that apply to some do not always apply to others so in this case he could have easly entered her room without her knowing or inviting him.
According to ancient legends, vampires are specifically prohibited from entering unless they are specifically invited to do so. Therefore, the "vampire" that appears in Sam's room could not have done so without her 1). knowing about it, and 2). her specifically inviting him in.