Greetings. Vaping is illegal in Singapore, But someone I know has never had an issue with it going through singapore quite often.
This person usually flies in to singapore, and of course there isnt usually a check going in to the airport... although it may be better to keep it in checked baggage and/or disassembled if really worried. Also in pockets... but rarely they do check coming out of the airport.. This person uses advanced mods, though, so they could appear to be other things. and as for going out of the country... theyve found this persons stuff in pieces quite often. RDA's, Most recently 60 mg of juice... theyve never had an issue with it since its leaving SG.
As for buying equipment... i believe its easier to pop over to Malaysia for that kinda thing.