I wouldn't say that's a vamp, they (usually stealthily) feed off humans.
That's more like a demon, which often make attempts to control humans and wreak havoc on earth.
The big concern i have, is that you have stated you have some spiritual / empathic sensitivity in this line;
I kept waking up the whole night after kicking the astral vamp out with an evil type of feeling in the air.
Except you have not stated where the evil intent is coming from. Is the being trying to "occupy" your body the source of the evil intent, or is it trying to occupy you so it can PROTECT you from the source of the evil intent?
Find that answer and you will have the problem resolved. Also i hate to burst your bubble, but your "willpower" has absolutely nothing to do with being able to kick out the invader. People can have plenty of will power and 0 spiritual growth/ability, when they are attacked they lose consciousness just as easily and as fast as someonoe with 0 will power. You either have a natural talent at controlling entities, or you have experience in doing so in prior lives / in between lives.
That is probably the reason for the attacks. They know you do not mentally understand your abilities and soul, so they see you as an easy target. They want to know how your abilities and soul work so if it really is a demon or other neg trying to invade you then you seriously need to learn to protect yourself.
Finally, no, this is nothing like astral sex.