Vanishing twin syndrome (lost at 5+ weeks), what is the likelihood of the remaining twin having something wrong with it?

Increased risk LBW. Compared to singletons, the surviving twin of VTS is at slightly greater risk of premature delivery, low birth weight and, for twin loss after 8 weeks gestation there is an increased risk for cerebral palsy. Loss of a twin at five weeks greatly reduces the risk for the surviving twin to have any adverse consequences.
Age Related. Risk of congenital abnormalities will be dependent on your age. At 41 years the risk is higher and more than 1 in 35 babies will have a congenital or birth deformity or abnormality. Genetic disorders increase with maternal age. Not related to vanishing twin.