meaning of sentence

Basically it's saying that the sentence fragment "the internet vendor are" is incorrect. It should be, "the internet vendors are," or "the internet vendor is."

To rephrase it:

If you use the phrase "the internet vendor are," expect that the people you are addressing will react with confusion.

"Momentary synaptic discomfort" is a fancy (unnecessarily fancy, but the author was apparently trying to be witty) way of saying that someone will be confused. It basically means that synapses (brain functions) wouldn't be able to function normally when trying to process something that is incorrect.

Thanks, Sozobe, for the lucid explanation.

Best regards