It's in Eb minor, main vocal I think for the 1st chorus line is:
Heard it through the grapevine
F# Eb F# Eb F# Eb
3rd is;
Longer would you be mine
G# G# G# F# G# F#
I think the main harmony notes are mainly Eb and G# for the lines 1 and 3 respectively, so the harmony is pretty much just the root note of the chord and the melody moves around the Eb minor and G# minor chords.