Evolve Gloom or Oddish

Depends. What do you value higher? The slightly better IV's, or the extra dust?

You will need 28 candys to raise your Vileplume from level 13 to level 20, and evolving your Gloom would save 25 candys. So, if oddishs are that rare where you live, i think you should evolve the oddish, since the IV difference is very small.

If the candy numbers I've read here are correct - evolve the gloom.

28 candies versus 25 is nothing - literally 1 oddish catch.
The gloom has higher IV. Not significant, but still something.

Then the important thing: moveset. You have a roughly 2/6 (so 1/3) chance of getting a grass/grass moveset. I'm assuming the other 2/3 are poor for you: acid/anything, or anything/moonblast won't make for a good water counter (which is what Vileplume is good for).

So if you evolve the Gloom and the moveset sucks? You can save the extra 25 candies towards your next one. I understand it will be ages but it's 25 less candies than you'd need if you evolved the Oddish and got Acid as a quick attack.

Now, if you're fine with any moveset other than Acid/Moon (so 5/6 chance at what you want) then maybe it's okay to do the Oddish. Even then though, I'd rather spend the 13K dust and 3 extra candies and have the extra 6% IV. I mean, it's such a small cost. Knowing the moveset before having to decide to power up is worth it IMO.

I guess the best way to look at it is to consider for yourself:
If you were guaranteed to get Acid/Moon on your first Vileplume, which would you want to have evolved? One that saves you 25 candies, or one that will default at higher CP to be usable? For me the obvious answer is the former so I'd evolve the Gloom, but I think you answer that question and it lets you know which to evolve.