This is an important lesson for inexperienced boat owners. Always check the dimensions and specifications of any part you replace.. use that info when looking for a replacement rather than using the year and make of the boat... especially a 45 year old boat! The boom vang pictured is likely not even original to the boat.. which is very common, since most owners will upgrade or add improvements to their boats over the years. Probably the most common assumption newbies will make is trying to find replacement parts for original electronics. Another misconception is thinking that older fiberglass production boats have some kind of "vintage" value... so newbie sailors will search far and wide to try find an "OEM" replacement for a piece of running rigging, a stove, a cleat or block, etc. Upgrading hardware, rigging and other appliances will improve the appeal and value of the boat. When people buy older boats the value is in the condition, upgrades, and evidence of ongoing attention to maintenance. The year built is not as important as one might think.
The irony here is that any part integral to the boat's design should only be replaced with identical, factory authorized parts .. such as the rudder, keel, centerboard, mast and boom... anything that affects the performance rating. Of course, the problem is that many, many fine boats are long out of production, the builder has shut down and the inventory of replacement rudders, masts, keels and centerboard is depleted. So finding such a part can be a nightmare if the new owner wants to race or maintain resale value. It will come down to fabricating the part, hopefully the plans are available...or finding a donor boat in a boneyard somewhere.... and it may cost more to deliver the part than the entire boat is worth.... That's why we all hate having to tell someone his "bargain" is missing a part that can't easily or inexpensively be replaced.
For what it's worth, owning a well equipped sailboat is an indescribably joy. I have loved working on this and my previous boats over the years.... but learned the hard way about design parts such as rudders, daggerboards and booms. It hurt the pocket book, but at least there was a supplier. Okay, my brother. re: the vang.... That looks like a piece of 3/16 or 1/4 inch line might fit your vang's jam cleat (there's an example of something you can upgrade, btw). Use the old line to measure, or a piece of inexpensive cord to create a pattern. Good luck.