Yes. Casein Protein Powder is allowed, in any amount, on the slow carb diet.
* Note: Casein Protein Powder is recommended as a substitute for Whey Protein Powder.
Vegan recommended protein sources are:
Pure Advantage Pea Protein Isolate
Sun Warrior Brown Rice Protein
Nitro Fusion Plant Fusion (Rice, Pea, and Artichoke Protein)
The above protein powders can be mixed with Almond Milk, and 1-2 Tablespoons of Almond Butter.