A half-step downtune may or maynot work. In terms of vocals, half-step is not a very big difference.
I would advice transposing the whole song with the aid of a capo. If the riff is too intensive and plays with the lowest notes such as the low E, you can try a drop D tuning. If it is still not enough, you can try to play the song one relative octave higher than the singing.
If all else fails, get a 7-sting. =D