I have the original VT75 and the VT75C. The C has some improvements that make it well worth it. I could care less about temp control, but the color display is really neat, and Ive got like 20 different themes I can upload to it now if I want. Its a lot more comfortable to hold than the 75 too. Probably the best upgrade to it though is the battery door. Its not that cheap zinc one that has bad threading that wears out like the 75. Its also way easier to screw in this time since they made finer threads on it (the 75 is a pain in the ...). Its actually all stainless now so it won't wear down from use like the zinc does. The base for it is also not part of the front bezel anymore, and a lot of the 75s bent (which gets worse with use), meaning the more it bends out, the more you have to screw the door in to make contact. Its also stainless and can be removed by two screws on the bottom should you need to.
The issues with it are obviously the 26650 problem. Because of the smaller size of it, you can only use certain 26650s with it, which are apparently all over the map as far as length. They need to be the right size otherwise when you screw on the battery cap, it pushes it all the way to top so the positive top pole will hit the body, which is the ground for the device. I only use it with 18650s so this has never been an issue for me, and I know the green eFest 26650s work fine with it so if I ever do want the longer runtime, Ill just get those.
Also, I don't know if its just mine or not, but Ive only had mine for about 8 months now and the paint on the front piece (the bezel is plastic unlike the metal body) has almost completely rubbed off now. Now my nice black mod is black with a grey plastic front. I kinda like the color contrast, but I never saw anyone else complaining about this so either no one mentioned it or I got a bunk one they used crappy paint on. The glue on the upper leather pieces on both sides turned into an oozing gel too and those started falling off (or moving around every time I touched it to pick it up and vape) too. I don't know if it was a heat issue or what, but it seemed like at some point the glue actually started expanding and oozing out of the sides of it and after that they moved every time you even touched them. Hasn't happened in awhile now, but I think thats because a lot of the glue has come out by now and Ive wiped it off so they're in place. If it does happen again, Ill just use alcohol and clean the back and use some gorilla glue or just leave them off.