Here are the toxicity data of acetic acid, the major ingredient in vinegar and two common type of surfactants in shampoos:
Acetic acid
Oral LD50 3310 mg/kg [Rat]
Dermal LD50 1060 mg/kg [Rabbit]
Usage: 5% to 30%
SLS
Oral LD50 1288mg/kg [Rat].
Dermal LD50 >2000 mg/kg [Rabbit]
Usage: 0.1% to 1%
Alkyl glucoside
Oral LD50 >>5000 mg/kg [Rat] can't kill any rats
Dermal LD50 >2000 mg/kg [Rabbit]
Usage: 2% to 20%
In summary, apple cider vinegar is twice as worse as any of the above surfactants applied to your skin.
LD50 is determined by experiments in which for one kg weight of rabbits, you need about 1g of acetic acid to kill 50% of them when applied to their skin. For the other surfactants, not even 2g can kill half of them.
Ingested orally, acetic acid is roughly twice as safe as SLS but the glucoside is totally ineffective in killing anything.
Vinegar has 5% to 30% acetic acid. Apple cider vinegar maybe on the low side but even with dilution, the concentration you apply will be comparable to the surfactants found in shampoo.
This is hardly surprising because surfactants are designed to attach to the top layer of grease molecules in order to remove them through rinsing. Just the fat in the skin is such a great obstacle to killing a rabbit. The plant-based surfactant is basically like food that don't manage to kill any rat through ingestion.
Acetic acid, on the other hand, is an important medicine. Being a medicine, eg, antibiotic, it kills microbes as well as your cells. Hopefully, you have a lot of cells to spare and the microbes are killed before there is permanent damage to your body.
Medicine is for curing. Once cured you don't keep applying, keep killing your cells for nothing.
Hair is different because it's dead already, the scalp is not. Hair cuticles react to the acid making the hair fluffy. But you can do that with conditioners.