I have a different understanding here noods. Since they're simultaneous in the order, I think the slayer's stake would still go through even if they were bitten. It only makes sense that way since it's a 50/50 if they target one another.
Think of it this way... if the slayer would lose the stake, then they would die; and it wouldn't come to a 50/50 shot. Maybe Labby could clarify?
The way I read the rules is if the slayer is the target of the vampires and the slayer is trying to slay someone it comes down to a coin flip to determine who goes through first. If the vamps win the coin toss slayer dies before staking, if the slayer wins flip he gets to slay his target then gets hit by the vamps (unless of course his target was the last vamp)