Meyerpires do not have fangs. This was mentioned by Bella during a conversation with Jacob (Eclipse, page unknown). Also, Bella had a crescent-shaped scar after being bitten by James, thus Meyerpires have normal human-like teeth. In that case, how do they suck blood? Animals who feed on blood don't have teeth similar to us for a reason. That is because sucking blood with teeth such as ours is impossible.
Predators use their fangs to puncture a hole in the body of their prey so that they will be drained of blood. If you have super strong teeth (but structurally human) and bite into something you will have to gnaw their flesh
like a zombie before getting any blood. Also, the blood vessels in the neck of a human are surrounded by thick flesh. Human teeth can't sink deep enough to reach to the blood vessel.
This is another proof that Meyer did not do any research and didn't use her common sense. Meyerpires are a disgrace to the vampire lore.
Or maybe they're just advanced zombies.