This is not actually as easy a suit to make
So true! I've been working on one since March, I've drafted about 4 different versions. I even took a short course on pattern drafting to get the fit right. The problem with a simple jumpsuit is that the top of the garment will need to support the bottom. If you use a heavy fabric, it will pull downwards and get saggy around the crotch area. If you're creating a suit from scratch, you'll need to add some proper construction pieces like a belt section. If you make it tight, the bottom will ride up if you raise your arms. It's like the top and the bottom are in a constant tug-of-war.
My first "Vaultsuit" was just a simple 101 jacket to get started, then I began to plan and research with a year run-up to my deadline to make a replica of the game model.
If you want to start basic, I've seen some pretty decent suits from Dickies and King-Gee.
Dickies Deluxe Cotton Coveralls are nice looking, the "bi-swing" back is also sometimes called an action pleat. This is a flap of fabric that makes it easier to move your arms around without pulling the entire garment. It's also used in lots of World War 2 era flight suits, which is perfect for the fallout universe.
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The KingGee Summer Weight Combination Overall is a lighter blue (a bit closer to Fallout blue, and could be dyed to the exact shade), it's brilliant for summertime because it's not super thick. This may be an Australian only brand, but you could check out eBay.
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I'm making my suit in two pieces, joined by the belt, this way I don't have to get half naked just to use the bathroom. :lol