Translation:Did anybody see you? No. Nobody saw me.
No, vay' is singular in both cases. It can be translated someone or anyone depending on the English sentence you're saying. (For instance, you have to use anyone in negative sentences: I see someone, I can't see anyone.)
So far as we know, you can only use vay' in the singular.
The prefix Du- means the subject is third-person singular (him/her/it) and second-person singular (you, singular). The prefix nI- means the subject is third-person plural (they) and the object is second-person singular.
nIlegh they see you (singular)
Dulegh he/she/it sees you (singular)
Previously, there was no way to create audio for the incorrect distractors, but we recently got a tool which might work to add audio to them. I'll try it and hope. Unfortunately we won't have any way to check and see if they worked unless you or someone else happens across the same exercises and comes here to say it worked.
You may have to wait two weeks to make sure the new audio has been fully integrated into the course. And there is no way to make sure that you come across this exact exercise again. That's why I say we have no way to check and see if it worked except if someone just happens across it.