That's true in declaratory subject-verb form, but in the case of vendre, the d in vendent is pronounced in declaratory form, so you can tell the difference. With other verbs, sometimes there's no difference between singular and plural in declaratory form.
In verb-subject pronoun question form, the t at the end of vendent and other verbs is pronounced, as both possible plural subject pronouns (ils/elles) begins with a vowel.
The real issue is whether in quesiton format, native speakers pronounce the d at the end of vend, and, if they do, whether they pronounce it as d or a t - or not at all. My exploration of on-line translators doesn't shed any definitive light on this quesiton.
Usually. but just to add some spice, the sentence may be in statement form with intonation raised at the end of the statement which turns a sentence structured as a statement into a question. Like "Vous Mangez?". " You are eating?"There are other, more formal ways of asking a question and your lessons will take you through all of them. Remember, though, when you invert the noun/verb to ask a question, you ought to include the hyphen between the inverted verb/noun as in my post. Bon chance!