As a tenor and singer, I hope I can answer this well! A tenor is a specific type of singer, typically a high-pitched, male singer. Examples are Luciano Pavarotti, Andrea Boccelli, etc. A singer is simply someone who sings. A vocal performer is someone whose career is to sing in a performance setting

From Wikipedia: A tenor is a male singer who has the highest male voice within the modal register. The typical tenor voice lies between the C one octave below middle C (C3) to the C one octave above "Middle C" (C5). The low extreme for tenors is roughly B-flat 2 (the second b-flat below middle C). At the highest extreme, some tenors can sing up to the second F above "Middle C".

Two useful online dictionary for solving such problem by Craige: "http://dictionary.reference.com, http://www.macmillandictionary.com" Thank you Craig!