The fangs, pale skin and red eyes are ever present yet no one pays any attention and calls it out. Not even the vampire hunter who is looking for him.
I think pale skin and red eyes could be explained otherwise, why shouldn't albinism exist in the Forgotten Realms? The fangs would be a dead giveaway for a vampire (as Astarion is definitely not a Tiefling), BUT: if there is one thing for certain for people in Faerun, it's that vampires don't run around in broad daylight. So Astarion might look like a typical vampire to Gandrel, but he can't be one, since he apparently has no problems with sunlight.
I think it would be the same as if we would see some animal running down the street that looks exactly like a T-Rex. We know it can't be a T-Rex since they are extinct, so most people would rule out that this is a real T-Rex and would search for another explanation.
