Er, no. Being airside in Changi airport is very much being in Singapore, and the possession of vapes and their components is illegal. That you don't bring them through customs doesn't change that (and you're maybe not likely to be caught, but if you are it's a fine: https://www.moh.gov.sg/news-highligh...-and-education )

BTW Singapore will also try to detect drugs and other illegal goods in transit - people have been executed for trying to transit through SIN with drugs (not that vaping is a capital offence obviously): https://en.m.wikipedia.org/wiki/Tong_Ching_Man