First crack on my original Vivant appeared today - just below the battery compartment. It has been dropped once, so that's possibly a reason - but my gut feeling is that it's just a shade too tight when you slide in/out the battery. It takes a bit of force to squeeze it in. Combine that with the battery being regularly swapped out on a daily basis.
Still, it's a hairline crack and well below any mouthpiece area - so a bit of Kapston tape to seal it and it will be fine.
Zero cracks in the mouthpiece area and never had any plastic taste.
Still a bargain for the price - but I would recommend putting some tape around that area now - it would also add some protection if you do happen to drop it.
The specific area is just below where the battery cover slots on - it's the entire plastic piece where the USB charging port is located - the sides (in my case it's the left-hand side) if you're looking at it from the front with the little green air vent switch directly in front of you.
I want to stress that this could still be because I dropped it - and the crack started internally and spread outwards.