I have a different boat…an O’Day 322.
A few years ago, I replaced my vang with a rigid vang Seldon). I can’t find a photo of the original set up, but it isn’t hard to imagine where it would go.
The Seldon rigid vang serves 2 purposes…1) to act as a bang and hold the boom down and 2) to support the boom and eliminate the need for a topping lift.
You can see that my Seldon vang also has a rope vang, but it is incorporated into the structure of the boom support
The original rope vang attached to a hard point at the base of the mast, and connected to a shackle on the boom (near where my new Seldon vang is connected. My rope vang also was routed back to the cockpit via a turning block and a rope clutch on the cabin top.
Photos of your existing boom and mast base may help us help you.
Greg