Medical procedures often induce physical pain and psychological distress. Cancer patients are particularly vulnerable to fatigue, heightened emotional distress, social isolation, and increased risk of depression, severely reducing quality of life and potential treatment non-adherence and prolonged hospital stays. Leveraging emerging virtual reality (VR) technology, we prototyped Serenity: a mobile smartphone-based VR cancer coping intervention informed by distraction therapy to help alleviate the symptoms of coping and promote patient empowerment. It leverages emerging low-cost VR to improve accessibility and effectiveness of distraction therapy. We describe the results of a preliminary usability study and lessons learned that apply to future design and evaluation of virtual reality interventions for behavioral health.