I think you put the vests on and then sleep suits if needed rather than just a sleep suit. I got confused by this too. It's better to have lots of layers you build up and can take off rather than a full on sleepsuit which might be too much for them
You put a vest on under sleep suits. When the weather is hotter (e.g. our bedroom can be 23 degrees at night) we were told to then use a vest OR a sleepsuit with our sleep bag. But the norm would be to wear both. I asked at my midwife appointment whether both would be too much for the baby today (it was 25 degrees by me) and was told no both layers are perfect for being out and about. X
They just need one more layer than we do, so if you aren’t actually comfortable in one layer of clothing and feel like being naked then baby would be fine with just sleep suit or just vest, but if you need a t shirt then they need a sleep suit and vest etc