I am trying to use my own symbol but everytime I try to use it, it says this:
You did not enter the correct format for the VIP: Username Prefix field. Please read the field description for the expected format.

There should be a list somewhere near where you can set the Prefix that tells you all the acceptable ones.

I believe you can use all of these http://www.utf8-chartable.de/

I was about to ask the same question. I was trying to use Ў this but it sais the same thing. Its in the list that u gave us.

Some symbols are unusable for normal players, such as "~" which only a few admins are allowed to use as far as I know.
If a certain symbols is not working for you, just try another one...