RecruitingPhase 3Drug trialA Long-Term Extension Study of Vedolizumab in Children and Teenagers With Ulcerative Colitis (UC) or Crohn's Disease (CD)The main aim of this study is to learn about medical problems (adverse events) if vedolizumab subcutaneously (SC) is given to a child or teenager with UC or CD for a long time. Other aims are to understand if the long time use of vedolizumab SC has an impact on the time period until hospital visits because of bowel swelling (inflammation) are needed and has an impact on the quality of life of children and teenagers who received vedolizumab SC. In this study, participants who responded well to the treatment with vedolizumab SC in the parent study (VedolizumabSC-3003 \[NCT06100289\]) will continue to be treated with vedolizumab SC. Participants who did not respond well to the treatment with vedolizumab SC in the parent study or who received corticosteroids in the last 4 weeks of the parent study will not receive vedolizumab SC in this study but will be followed for up to 2 years after the last treatment with vedolizumab SC in the parent study. During the study, participants will visit their study clinic several times.