For the vivi's and the protanks (same for the evod's) I use 30 or 32. I find 28 too thick, unless you want really low ohms and want to put it on a mech.
For VV/VW I would stick to 30/32g wire.
Doesn't matter what kind or wick you are using. I agree, 3mm always works for me. I tested with double folded 3mm last week. It works, but I got too many wraps on there, so the resistance was on the high end for me. On the protank I do not use flavor wicks and not a sillicon thingy on the air tube. Gives me more flavor. Haven't tried this fat a wick on the vivi though...
So anything from 3 - 4 X 1mm - 3 X 1.5mm - 2 X 2mm - 1 X 3 mm etc. will work. Just use what you have lying around or if you are just starting, try out what kinds of wick you like the best in terms of making a new coil and wicking capabilities. If you are tight on a budget, try to see if you can recoil with el-cheapo cotton. Look for the
threads here on ECF. It wicks great. Small learning curve. I prefer German silica right now, however, is more expensive then cotton balls or cotton roll.
The most versatile wire for clearo's is def. 32g I think.