If vox populi is the voice of the people, (or people's voice)what is the proper way to say... people's response...
“Responsum populi” is the correct translation for “People's response”.
Also, you could say “Populi responsum” with a different word order that in Latin can be variable.
Please learn more below.
As for "Vox responsi", it is wrong as it would mean "The voice of the response" and then would not correspond to “People's response”, of course.
-People’s = POPULI (genitive singular of POPULUS, masculine noun, 2nd.declension)
-response =RESPONSUM (nominative, neuter noun, 2nd.declension)
As for “Vox populi, note that VOX (nominative, feminine noun, 3rd.declension) means “the voice, while POPULI (genitive singular of POPULUS, masculine noun, 2nd.declension) means “of the people”.
Finally note that Latin word order can be different from English as Latin is an inflected language where syntactical relationships are indicated by the ending, not by the order of the words.