if you're getting burnt flavor, your juice is probably too thick to wick correctly, or the head is gunked up. up until a month ago, my VN ran on nothing but 100% vg, and had an issue the first day with burnt, dry hits. cut 1/2 of one of the sides of the wick off. havent had an issue since.
its a $12 tank, its not going to be perfect, or run flawlessly without any fiddling. the ONLY piece of vaping equipment that i've seen run flawlessly so far is a Provari, and it was $200......
the 4-5 months i've been using a VN has MORE than paid for itself. usually burn out a head every 3-4 weeks, replace it, and its perfect for another 3-4 weeks. im not going to complain at all for $1/week maintenance fee. every other tankful, i usually tear it down, wash it out, rinse the head, pat the wick dry, put the head on the base without the tank, and dry burn it until the wick is dry and the coils un-gunk themselves. usually this is when the coil fails, i'd rather have it burn out with it torn down and empty, than waste a tank of juice.
mines a version 3, by the way. and an actual vision vivi nova, not some chinese knockoff that places are selling for $7-8..... that could have something to do with problems too.