"Vendor(s)" is something of a fake word that is deliberately ambiguous about number, and so it inevitably creates problems with the rest of the sentence.
In general, if you're not sure whether the number is singular or plural, I think the convention is to use the plural. So you could just say, "Buy an apple from the vendors that are selling fruit." Like if you say, "Interview all the applicants who have at least 5 years experience", it is not normally considered necessary to reword the sentence to allow for the possibility that there will be only one such applicant.
When it's necessary to be clear that the number might be singular or plural, the grammatically correct thing to write is, "Buy an apple from the vendor or vendors that are selling fruit." Now the uncertainty goes away, because there's a clear rule of English grammar that when you have a noun phrase with "or" in it, the number of the verb must agree with the number of the LAST noun in the list. The only time I do this is when I really need to be clear that there might be only one, if for some reason this is not obvious from the context.
When people want to be more concise, they sometimes write things like, "Buy an apple from the vendor(s) that is (are) selling fruit." Or, "But an apple from the vendor/vendors that is/are selling fruit." But I don't know of any authoritative source to back this up as legitimate, and it looks rather awkward. If the sentence gets long or turns into a paragraph, all the parentheses and slashes could get pretty tedious.