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You need a definitive diagnosis as to the issue before treatment can be given. The treatment and disinfection for mites and ringworm are different.
Why did the vet think ringworm was unlikely?
In the early stages mites and ringworm can look the same so if it was on look alone then that can be a tricky thing to call!
You will know quite quickly if it is ringworm though because it will spread to both guinea pigs, and to you - so ensure you take precautions when handling them just in case. See ringworm guide I’ve added in below
Did the vet give you mite treatments?
I ask this because ivermectin treatment has to be done properly as part of a course (three separate treatments with two weeks between each treatment (although this gap is product specific) to ensure the full life cycle of mites is caught (treatment doesn’t kill the eggs, it only kills the hatched mites so there has to be the repeat treatments to ensure no mites remain to continue the cycle).
Treatment has to be dosed properly for their weight.
Pet shop products are too low dosed to properly cure a mite infestation.
Bathing will not resolve anything on its own; and you have to be careful about doing it, particularly if you’ve treated them with ivermectin.
Ringworm would have to be properly diagnosed and then the treatment prescribed by the vet. The best treatment for ringworm is a prescription only oral medication (I can’t remember what age they have to be for it though).
Using dips to treat ringworm is the next best treatment if oral medication isn’t an option.
Creams are not effective as they only treat a localised area but ringworm spores are shed from a wider area than you would cream. Meaning the infection is very likely to continue to spread.
There is also environment disinfection to consider whether it is mites or ringworm.
We can’t diagnose anything but adding the picture would be helpful.
I have added two guides below which explain piggy parasites: hay mites, mange mites and lice.
Lice are visible to the human eye as pale crawling creatures.
Mites (both types) are not visible.
You will spot hay mites as you will find tiny beads stuck to the hair shaft. There are egg casings which won’t come off. You have to cut the hair off.
Mange mites live in their skin and are more serious than hay mites. They can cause a v shape bald patch on their back end.
New Guinea Pig Problems: Sexing & Pregnancy; URI, Ringworm & Parasites; Vet Checks & Customer Rights
Ringworm is usually bald patches, possibly bleeding from the scratching, white crusts on the skin (although bathing will unfortunately hide those defining markers). The guide explains the treatment but also the strict hygiene which would need to be taken
Ringworm: Hygiene, Care And Pictures
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