What's Good for Your Sole
Overall, the motion or action can help completely engage the side of your body that is facing the mat during side plank to act like a supporting arch to hold you up. This is a line of fascia known as the lateral line (pictured to the right).
So to answer the question of whether or not to evert or turn your foot out in Vasisthasana, just do what feels right in your sole!
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Side Plank Photo: https://s-media-cache-ak0.pinimg.com/originals/d5/76/0c/d5760cb2d30169703765000b4faa8749.jpg
Ligament Images: Thieme Atlas of Anatomy: General Anatomy and Musculoskeletal System
Lateral Line Image: Anatomy Trains by Thomas Myers