Grammar: vacancy

Hi,

I was told that the third sentence is the best option. However, I was wondering if I could use the other four sentences as well.

1) "We have a vacant position of an assistant."

2) "We have a vacancy of an assistant."

3) "We have a vacancy for the position of an assistant."

4) "We have a vacancy for an assistant position."

5) "We have an open position for an assistant."

For instance, the first one and the fifth one both sound quite good to my ear as well.

What do you think?

I'm sorry to bother you with this.

Thank you.