Aih, this must be such a stressful time for you, detecting a vanishing twin situation at scan is really emotional. A few of us mums have been through something similar, and honestly the uncertainty is the hardest part, tapi you really need to see your doctor soon to understand what's happening and get proper monitoring, because sometimes the smaller twin does catch up, sometimes it doesn't, and a doctor can explain what to expect for your next steps. The good news is the other one has a strong heartbeat, which is very positive, so focus on that one and let your doctor guide you through this carefully, they may want to scan you again in a week or two to see how things develop. Fingers crossed for the best outcome for you!