Kiss My Face's Sahaira shampoo is supposed to work for dandruff. I have some, but haven't tried it yet because I'm finishing some other shampoo first. Anything with tea tree oil is supposed to be good for dandruff, too.
Here's a dandruff treatment from How It All Vegan:
2 tbsp apple cider vinegar
2 tbsp water
2 tbsp flax seed oil or olive oil
Combine the vinegar, water, and oil in a clean, dry container. Shut tightly and shake until combined. Massage onto scalp and leave in for 15-20 minutes. Wash out.