This looks a bit dangerous as the plate on the bottom on the mast is clearly over loaded between the vang, Cunningham? and 2 reefing lines(looks like single line reefs running aft). To answer your question, no the vang should not be attached to the deck plate for the mast. Where the vang is currently located appears to be correct however everything else overloading that attachment point needs to go. It also looks like the mast plate has been distorted after the rivet failed. Pull the plate off, flatten it, drill out the old holes, tap the holes and use new machine screws to attach it. I would also relocate the Cunningham and reefing lines if possible.
Mainsheet running from the outboard end of the boom, down to the traveler, back up to the outboard end of the boom, down the boom to mast base and back to a winch and stopper on the cabin top is common for high load race boats. I’m not sure yours would qualify but if you have enough line and good blocks I would keep it. Note, this setup usually requires a good crew as the helm can’t adjust the main sheet from the wheel.
“Greatness Is Not In Where We Stand, But In What Direction We Are Moving. We Must Sail Sometimes With The Wind And Sometimes Against It – But Sail We Must, And Not Drift, Nor Lie At Anchor.”
Oliver Wendell Holmes