The easiest way is buying this from Bintel.
http://www.bintel.com.au/Accessories...oductview.aspx
That way you can keep the vixen bar as you have it. Better and cheaper is to attach the rings to a wide Losmandy bar but you loose the adjustable screws…if you use them that is.
http://www.bintel.com.au/Accessories...oductview.aspx
I'd go the Losmandy wide bar myself.
Matt