Vain in this usage means worthless!

True Love is Vain until it Hits a Vein
Oh, baby
My love for you is as long
As this song
And I know it can’t be true
Cause I’m done

I actually sung this before I wrote it at a party, that time of the party when it's pretty much over and everyone is just sitting around tripping instead of rocking their asses off. So with my buddies acoustic I was singing some shit, and the girl I was with that night told me to sing a song for her. So, with a big long beautiful song planned I tried to belt some shit out, but only got to the line "As this song," when I ran out of words and thoughts, so I ended it accordingly.

As drunk as she was, she loved it.

Adieu,
Zak Hunter