Is it possible that a vodyani population grows on my (Vaulter) world ? I thought they cannot mix... I assume it is a bug. What do you think ? I can move it through the planets like any other guy...

There were some vodyani ships in orbit, I saw that the next population to be grown would be vodnyai and now, I effectly have a vodyani guy... and he reduces my food production by half... (... yes, he does that !)

Can i .. hum... kill him ? I tried to convert him into 300 worker force but despite the correct icon, it's somebody else that has been consumed... 

It's not a bug. Vodyani is just like any other pop(you can move it, kill it, grow it, etc.). U can invade vodyani ark and capture the system and ark's Vodyani population. Also you can get a Vodyani pop through curiosity. The icon just glitched prolly that's all.

just put it in orbit to go to another unassigned system

Ok, thank you ! I learned something !

You should be scared, if you saw one Vodyani, be certain the rest are coming to purge the unfaithful! :D