Isn't enlightenment the ultimate death? Is it true that after you reach enlightenment you simply vanish into the vast space of existence, and you simply cease to exist?

Kind of like, when you have not attained to enlightenment you keep on returning and being reincarnated, but if you reach enlightenment and stop being reborn don't you just cease your existence and die forever?

If I remember correct the Buddha did not know what happens to an enlightened person after his death, or am I wrong?
By asking if a person or someone reaches Nibbana is implying that people/persons exist meaning that one is not dealing with ultimate reality but instead with conventional reality.

In ultimate reality there is no person or being that gets enlightened.

Take the "i, me, you, him, her, it" out of the sentence and just observe what is left -> Enlightenment happens. Enlightenment is reached. 

The unconditioned, Nibbana cannot be understood by conditioned language or logical reasoning. In other words the unconditioned cannot be grasped or understood by using tools from the conditioned realm.

The unconditioned must be experienced and comprehended through insights into reality.

If another view is taken which is that when enlightenment happens the self is destroyed, that view is flawed too. Why? Because there didnt exist a self in the first place so there is nothing that gets destroyed. Instead what happens is that it is realized through insight meditation that a self is nowhere to be found.  

In this book "What Buddhists Believe" by Ven. K. Sri Dhammanda, on p. 70-74 there is a chapter about what happens to a Buddha after death. Its a conversation between the Buddha and Ananda where 4 propositions are mentioned. They are;


A Tathagata exists after death;
A Tathagata does not exist after death;
A Tathagata exists and yet does not exist after death;
A Tathagata neither exists nor does not exist after death’


A Tathagata is a Buddha. You can read a bit more about the meaning of the word here.

Lanka