Another "Which one" question

How dare you speak of Porygon in such a derogatory manner.

Porygon2 is decent. Porygon-Z rocks your socks off. It's also incredibly rare outside of raids (meaning IVs are easy to get but candy is gruesome).

Slaking, as I've often stated before, is only useful as a supplement to Blissey gyms at full motivation to defend an EX-Raid gym while the egg is about to hatch. It's only useful when well-fed and only to serve as a non-Blissey meat-shield.

I would wait for a better Slaking. You'll almost certainly find a level 34+ one at at least decent IVs. IVs only matter if you're going to power something up. Now I know IVs seem more tempting on something as hard to get candies for as Slakoth, but I wouldn't go so far as to suggest that you power up a <90% Slakoth all the way from level 20, in addition to evolving it. Are you really going to spend very uncommon candies and loads of star dust for a gym stuffer? If so you must get lots of EX-Raids. The only real benefit a Slaking has in a gym over other defenders is that they are often fed, but this can be said of defenders like Snorlax and Blissey as well since they too are uncommon.

In my opinion, what it comes down to is whether you just want Slaking as a Dex entry or if you intend to throw it into gyms (please don't ever attack with it. Trust me. Mistakes were made).

If you do want one to toss into gyms, I'd say wait on a big ol' weather boosted Slakoth. Sure, Slaking isn't exactly a stellar gym defender, but it's fun. Most of the fight, it'll just sit there doing nothing (Yawn), but once it fires off a charge move (TM to play rough, it's worth it), if your opponent doesn't dodge, their Mon basically gets one-shotted.

The weatherboosted one. Slaking is a decent defender, but I would strongly prefer not putting dust into it.

I feel your pain, 600+ Porygon candies here also. Those things are a nuisance.

Doesn't matter. Slaking is a vanity mon, just good for saying you have something over 4000 in your pokedex. You can put it in a gym if you like, but it's not that useful there, or for anything else.

I'd evolve the level 32, and keep your eyes open for weather buffed ones. I got lucky and caught a level 35 at 89%, so I'm done with that.

All that said, Vigoroth makes for a decent gym defender, active unlike the other two, and adequate movesets. There are clearly better choices for gyms, but if you treat gyms like I do, putting in first stage evolutions and then transferring them to the professor when they come back KO'd so I don't spend revives on them), it's a fine choice, just like Pidgeotto, Raticate, Noctowl, Ledian, Ariados, Fearow, Furret, Linione, Golduck, Azumarill, and so forth.