The higher the PG content in a juice the less "visible" vapor is produced, nothing harmful about vaping 100% PG juice if you're not sensitive to it and can tolerate the taste. Single coil attys are best for stealth. For mechs build higher ohm coils (1.8 ohms and above), for regulated mods use lower wattage settings. If you DIY increase the nic content of your juice so that less frequent and smaller inhales provide the nic you want/need. Avoid vaping flavors/mixes that produce a lingering distinctive smell, unflavored is best for maximum stealth. Avoid vaping in close proximity to others. With practice, common sense and the right gear you could vape pretty much anywhere undetected.