Sylveon, Weezing, and Qwilfish are some good counters for Sirfetch'd.
Those metagame trends have recently made it a lot easier to replace on teams, although now it's the only reliable Sirfetch'd switch in, meaning that I believe in time it'll potentially overtake Vap again. That's why I'm split on where I would rank it. Atm, to me it's A, but in the next two weeks or so, I see it being A+, especially if Hail leaves soon.
Sylveon is one of the few reliable switch-in to Sirfetch'd in NU, that gets reliable recovery in form of Wish. It's able to beat most Fighting-types, a Dragon-type switch-in and acts as a good cleric with Heal Bell. Though Vaporeon and Diancie provide same Wish and Heal Bell support respectively, their typing isn't as good to deal with Sirfetch'd. Here's a sample moveset for it:
Sylveon @ Leftovers
EVs: 252 HP / 252 Def / 4 SpA
- Hyper Voice
- Heal Bell / Mystical Fire
Honestly, the only drawback I've experienced playing with this mon is that it really requires a balance/defensive team. With the rise of Fighting types, Weezing is here to dominate offense all day. Being able to wall so many scary offensive mons, especially Sirfetch'd given the right tools, is invaluable right now. I can't really describe in words how satisfying it is to watch most Dual Screens and Terrain teams just fall apart since you can lay Toxic Spikes while forcing out a lot of physical threats. After the mandatory Sludge Bomb/Toxic Spikes/Will-o-Wisp, you can run Rest for recovery (I find Vaporeon to be an excellent partner), Pain Split to act as a stallbreaker, coverage, etc. And then there's the whole Neutralizing Gas thing to absorb Toxic Spikes yourself and set them against Espeon and deactivate power boosting abilities.
Here's a set for it:
Weezing @ Black Sludge
EVs: 252 HP / 160 Def / 96 Spe
- Sludge Bomb
- Toxic Spikes
Qwilfish sets itself apart from the two poisons I mentioned before, in that it alone has tools to beat Entei and Glastrier. It has the move pool to beat them both, and still get spikes up before it is done. I have been running a set with Spikes/Liquidation/Taunt/Painsplit, and it ruins fat builds with mons reliant on wish passing from Vaporeon. It can run Taunt to beat ID Glastrier or Haze for SD or Vintei variants. Really solid mon with a great defensive typing, deserves much more credit than it is getting. It can also EV to be faster than Tyrantrum or Sirfetch’d netting surprise KOs on weakened foes.
So, these are few options for your team. Weezing if you need Toxic Spikes support, and as you already told in DMs that you have Vaporeon as your Wish Passer. The post also recommends using the combo with Vaporeon. Sylveon isn't much needed, use it only for Heal Bell support. Qwilfish if you need for Entei or Glastrier, but I'm supposing Vaporeon overlaps with it.
Hope this helps!