All it would take is for vampires collectively as a (sub)species to really want to stay undiscovered.
All it would take is for vampires collectively as a (sub)species to really want to stay undiscovered. In most lores, they're powerful enough beings that it seems if each one was convinced of the necessity of staying a secret, they'd be able to with relative ease.
In what situations would a vampire be examined by a doctor? Here are some I can think of:
A vampire murders a human (to drink their blood for sustenance) and is caught by the police and sent tomedically examined in jail. This seems like it could be avoided rather easily byIf the vampires. For example, if they have traditional supernatural powers, such as the ability to shapeshift, or if they have enhanced fighting capabilities (greater stamina or strength than humans)and stamina, they'd be able to escape such physical capture with relative ease.
A young vampire is going to school and, in an effort to seem completely normal, volunteers (like the other kids) to be examined by the doctor for one reason or another. The doctor, surprised by strange and unusual features of the vampire's body, contacts the young vampire's parents in order to request permission for further study of the peculiarities. The vampire parents realize what will happen and they whisk the young one away before the doctor's testimony can be corroborated. In the worst case, they change their names and move to a new town, state, or country.
A vampire is caught in a serious accident and is rendered unconscious. Bystanders take him/her to the hospital and doctors, examining its wounds, discover something unusual. The doctors of the one hospital start talking about a vampire, but either
a. when/if the vampire regains consciousness, he/she would use his/her supernatural powers to run away, or
b. when the other vampires of the community find out that this one's secret has been discovered, they whisk the still-living vampire away from prying eyes, destroy the body, or silence any witnesses.
Of course, these aren't all the situations where a vampire would be examined by a doctor, but since vampires don't have the same medical needs as humans, they would need to either want to reveal that vampires are real, or be involuntarily exposed to a doctor.