To answer your TPN/PPN question, I am assuming the difference is that TPN stands for Total Parentaral Nutrition while PPN stands for Partial Parentaral Nutrition; total meaning the patient receives no other form of nutrition while partial means they are receiving another source of nutrition probably via feeding tube. If they were receiving TPN, there would be more nutrients in the mixture than PPN, because the doctor factors in nutrients from the tube feeding as well (just guessing here, I've never personally seen PPN orders, just TPN).
As far as it going to the central line and not a peripheral: A central line is placed to end in the superior vena cava. This is a large vessel with a large amount of blood flow, so it can handle caustic fluids being infused into it. A peripheral line is much smaller and has a lot less volume flowing through at any given time, so when caustic fluids are infused, it is very damaging to the vessel wall and can cause a lot of complications. The reason why TPN is caustic is because it contains a lot of packed particles, i.e. glucose and several minerals + electrolytes. Scientifically speaking, the osmolarity is too high for a small vein to handle. There are other drugs that are central line specific.
I don't know the answer to your vanco question because I've never heard that vanco can't be infused into a central line (and that's why its a good idea to look up administration details on IV meds, you don't always know these details by memory). I've never given vanco through a central line, but I'll definitely be looking that one up because a lot of my patients have PICC lines.
Hope I helped to answer your question