bittersweet - ah yes, oxymoronic language is a favourite for writers and composers enchanted and/or enchained by love. I always liked the idea of 'slaving to need'. And there's is of course the concept of a fine line between pleasure and pain so often used.
I'm not sure where the 'sweet' part comes in during 'Vincent'. It has a certain melancholy resolve that we've come to admire from a suicidal protagonist though.
As for some tunes that spring to my mind, one is the Bristol outfit Portishead, in particular the song 'Glory Box'. Their music reflects the slippery bittersweet state well through the employment of a melodic song, but coupled with the use of highly syncopated beats, interspersed with scratching and discordant samples, and to top it off, a lady's voice that ranges from quiet whispers to long, loud, lazy notes that fall to bits. Good stuff anyway.
For anyone that likes open-tuning and great songs there's always 'The Red House Painters' (from 4AD label) with the sweetness in the music and the bitterness in the content. Powerful yet overwhelmingly simple guitar music. Probably something for those who like Dylan or Ryan Adams, who would both deserve a mention in this topic anyway I guess.