Good idea about the vinegar.
You'd only need a tiny amount of detergent for 8 nappies. For maybe 16 nappies including 2-3 big night nappies I would use 5ml (a teaspoon!) of liquid Persil non-bio. That's one seventh of the recommended amount, so really tiny! Everything is perfectly clean, I do a rinse before and two rinses after the wash, which is a 30 degree superwash (2.5hour) cycle, with the occasonal 60 degree wash. Even with that tiny amount of detergent and the extra rinses, I still need to strip maybe every 3 months but a quick detergent strip works fine.
So, if you're using powder I don't know how much it would be but I would start by looking at how much the manufacturer says you need for normal soil and divide it by 7. It's easier to tell if they're not clean and rewash them in a bit more if needed, whereas as you've found it can be tricky to get rid of build up, so start with a little and increase it if needed. You need to start with sparkly clean nappies first though. Oh, and I'd say that 8 nappies is half a nappy load, so divide the amount in 2 again. So if it says 120g (sorry if that's way out, I have no idea about powder!), divide by 7, then by 2, giving you a grand total of 8.6g of powder!!!
ETA - I'm quite exact about what I use. For the first time, I'd actually weigh it out! Then I use an old formula scoop to get the same each time, or a spoon or whatever - just make sure you can remember how dull it was
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