The setup BlowMeAway has is ideal. Best is when a single bolt or pin goes through the whole spar. However, a caution:
DO NOT try to install this without pulling the mast off the mast step first! Till you do, you may have no idea what you'd be drilling through. And, as was stated above, the harder you trim the vang, the more downward pull you get on the boom - but also, the more outward pull you get on the mast. So sheet-metal screws and 'just threading into the aluminum' is not the best way to go (by far). It should really be a through-bolted bail on each end.
The very
worst thing to do is to use something like the 'misshapen' pad eye on the deck. Just:
NO.
Do not
ever impose upward rigging loads on a cored deck. There is no need, no benefit, and definitely no security. In fact, remove that piece of junk now and dutifully fill all those holes with epoxy. In fact you'd better inspect it to see if you might not need more remedial work there. This is one place were soggy core and compromise in any other way is just plain bad news. It's not your fault - that vang should never have been attached there - and
please do not use it again!
One really cool trick is to wriggle a stainless-steel padeye into the mainsail boltrope groove, where it's extending down below the boom towards the deck (if yours does this; mine does). But this will have to be heavily bolted (typically by just threading the aluminum), so use the biggest padeye you can find that will slide in (because the bolts will likewise be big). You have only a H27, so the main is not that big; and remember with a short boom like you have the leverage on a vang - let's say extending out over a quarter of the boom's length - is pretty helpful. (If your boom were not as short and the same height off the deck it'd be less leverage.)
On my boat I fitted a Garhauer halyard-block tray under the mast step (please don't spread that around; but it is the
only piece of Garhauer on the boat). This is beyond ideal because it keeps all the upward pull of halyard blocks under the much greater compression load of the mast... essentially negating them as far as the deck is concerned. There is
NO better way to locate halyard turning blocks, vang fittings, reef lines, pole lifts, etc., than this. Everything else is a structural compromise.
If you're not unstepping the mast, just take off the boom & gooseneck, slide the padeye fitting down the groove til it's just off the deck, and then drill pilot holes before sliding the fitting out of the way and finishing with your metal tap. My trick has long been to put white electrical tape on the back of the fitting before screwing it down - this keeps the two metals nominally isolated and prevents (or at least long forestalls) bimetallic corrosion. Then use that bail for attaching the vang tackle.
No need to go crazy on snap-shackle blocks if you're leaving the vang in place all the time. Also one end does not have to have a swivel shackle (have the swivel at the bottom). I buy oodles of used blocks on eBay - you'd be surprised at the savings. Stick to good Schaefer ones since that's what your boat had when new.
