TL;DR -- Neither was anywhere near as good as vaporizing diphenidine
Though there are virtually no reports of vaping ephenidine, and not a lot of info with MXP, I have heard both vape well. Unfortunately this was not so. MXP produced effects quickly, but it was WAY too stimulating for me. After a couple of tokes, I spent 20 minutes trying to take deep breaths while my heart was beating out of my chest. I took a gram of phenibut in a desperate attempt to calm my body and mind, though I couldn't find any reports of anyone mixing them, and that did help a lot, but within an hour I was mostly just high on phenibut. Friend who is a stim fan (I can't handle them) enjoyed it, and finished that bowl for me. He had already taken phenibut, so could have been a factor. We both agreed it was very stimmy and not so psychedelic. He compared it to methy MDMA.
Ephenidine produced better effects vaped, but was difficult to vape, and took many many many tokes to finish 50mg. I ended up vaping 3 50mg bowls (with a friend). The smoke tasted quite bad. (the MXP actually tasted wonderful -- like flowers smell) It also gave me a sensation that my lungs were filling with fluid by the time I was getting good and high... no bueno. Unlike the MXP that left some nasty black residue, ephenidine vaped totally clean, leaving nothing behind but white cloudy stuff in the stem. After finishing all that over the course of an hour, it did produce a nice high with some psychedelia for the next ~3 hours, and stimulation for the rest of the day that was not unpleasant, but I concluded that it wasn't worth all the material needed to vape. I don't think I could have reached an "E-hole" with this ROA. I was surprised to find that that night, though I felt totally mentally and physically exhausted, I still couldn't sleep at all, even after loading up on magnesium and drinking some sleepy tea. My chest started feeling tight when I layed down and I got all sorts of troublesome body twitching throughout the night, taking breaks every couple of hours to get up and move around a bit, which helped those symptoms to subside by morning. No sleep, though.
Definitely taxed my lungs. Wasn't worth it. Will not be repeating. I'm not even sure I would want to take a large enough oral dose of either one for a full blown experience given how stimulating both were (ephenidine was tolerable, but MXP I don't even want to touch)
I was mostly interested in another disso that would vape like diphenidine (Deep, mysterious mind fuck that I cannot recommend enough -- only requires 30-50mg, which only takes a few good tokes, and the vapor is pleasant)
Sadly did not find it in MXP or ephenidine.