In my 'project' unit, the OPV was not in great shape. It was stuck, resulting in no moderation of flow. The internal 'nub' was all mucked up, and essentially un-cleanable. I happen to have a few brand new nylon pickup selector knobs for Stratocasters laying around. So I cut as much off one as I could while not compromising the interaction with the spring. It seals against the inside perfectly.

Given that the 'official' part in question is nowhere to be found on the market, this solution will work.

The teflon seal will be the next to fall apart if the valve has been over-tightened too many times to force it to seal. Just an assumption as to why your knob was ruined.
Part #9 in the "kit"

https://coffeesection.com/products/saec ... t-16-piece

Here it is with the new pump, and the pack of selector knobs

Great job coming up with a repair piece!

I've seen a sealing piece like that called a "mushroom" and there appears to be different sizes of it. There's a tiny one inside the brass outlet fitting on your vibe pump. The Isomac Giada uses one inside the Group Head. The dispersion "nut"(actually a brass bolt) holds the shower screen in place, but there's a spring pushing a silicone mushroom against the boiler outlet tube. Our friends at Stefano's sells them:

Giada (early) Mushroom: https://www.espressocare.com/products/i ... room-valve
Giada (after 2011), Maverick, Maverick Plus Mushroom: https://www.espressocare.com/products/i ... room-valve
Saeco Mushroom: https://www.espressocare.com/products/i ... room-valve

I opened up the functional OPV on my second ViaVenezia. The 'mushroom' piece is what's in the attached pic. The head is hexagonal, with a flat top! IMO not only is it over-engineered for the function, but I'm not at all surprised it squeals so much in operation.

The one I made does not squeal.