My daughters room is 21C and just has a babygrow x
I'd say a vest and sleepsuit is fine. Rule of thumb is one more layer than what you would wear. Failing that, you can always check if they're cold by putting the back of your fingers on their chest and layering up or down based on that x
My little boy is wearing a vest, trousers and socks and is plenty warm. For sleeping he's in his vest and 1.5 tog as his room is 20° tonight