Students experiencing pain or restriction in the spine, joints, and supporting muscles will benefit from a systematic approach grounded in the principles of Iyengar Yoga. Postures are adapted using props to meet individual needs.
Practice implies a certain methodology, involving effort. It has to be followed uninterruptedly for a long time, with firm resolve, application, attention and devotion, to create a stable foundation for training the mind, intelligence, ego and consciousness. ~ BKS Iyengar
Vrksasana is a Level I pose. Utthita Ardha Padmottanasana (Upright Half Lotus) is a Level 2 pose and is a harder variation of Vrksasana. Can you see the resemblance? It is in the same family of asanas, lateral standing poses.