This is a question I wonder as well. When I was a meat eater. I lost 50lbs on on 20-30g carbs per day and kept it off for 3 years. Then I became a vegan and I thought the weight wouldnt creep back on but it did. Now I am fighting to lose the weight again but I have no idea how low is actually feasible and if you can even get into ketosis as a Vegan. I'm going to shoot for 40g per day and see how that does. I figure its a dramatic reduction in what I had been previously eating.