As someone who loves to try out random gimmicks, I actually had given Eviolite Ivysaur a try a little after Amoonguss left. The set I used was generally Sleep Powder / Giga Drain / Synthesis / filler, with the last slot being Hidden Power or Sludge Bomb, depending on what I was testing at the moment. The EV spread I tried was 252 HP / 176 Def / 80 SpD with a Calm nature, which gave me enough physical bulk to always survive CB Sawk's Close Combat after Stealth Rock, and the rest was piled into special bulk. It's actually pretty darn bulky with the Eviolite, having approximately the equivalent defenses of an unboosted Pokemon with 60 / 115 / 134 defenses and equal investment. Just to compare this to Vileplume's bulk, let's say you have a Vileplume designed to hit that same defensive benchmark (surviving a 2HKO from CB Sawk's CC after Rocks) with the rest used to bolster special bulk. This spread would be 252 HP / 144 Def / 112 SpD with a Bold nature. To compare the special bulk of these two, Vileplume is taking 65.25 - 77.11% from Timid LO Samurott's Ice Beam, while Ivysaur is taking just 49.07 - 57.71%. This goes to show that Eviolite Ivysaur does have a great bulk advantage. It also didn't miss the Leftovers all that much since it had Synthesis, which was a neat asset.
However, I ended up switching back to Vileplume anyway for a couple of reasons. For one, Ivysaur's lower SpA stat was bit disappointing at times. Ivysaur's maximum damage rolls tend to be equal to or less than Vileplume's minimums, which kinda hurt when trying to take on Water types and such. The extra bulk turned out to not be all that helpful either. Vileplume could still hit those important physical bulk benchmarks (which are more important, in my opinion), and even with all that extra special bulk, Ivysaur is still usually 2HKOd by LO Samurott's Ice Beam and always OHKOd by +2 LO Gorebyss's Ice Beam. In the case that either one is still going to be 2HKOd, I'd rather be able to do more damage back and heal a bit with Leftovers. As Punchshroom mentioned, there's also Aromatherapy to take into account, which is a pretty good utility to have. Vileplume is one of the few Pokemon in NU to learn it, and many of the others that do are either mediocre in the NU metagame or have little room for it.
Overall, I'd say that Ivysaur isn't all that bad, but Vileplume generally overshadows it.