How often are vsd's missed as child and not found until adulthood? rare?

Unusual: It's not all that uncommon for a vsd not to have much of a murmur on routine exam as an infant/child. However, when a child is sick, etc, one can often pick up an unusual murmur consistent with a vsd, and the defect is discovered. In unusual instances, say the kid never gets sick, or at least a doc familiar with vsd murmurs has not seen him when sick, the lesion can indeed go undetected.

Rare: A large vsd which causes an 'overflow' blood pattern can lead to overcirculation heart failure. That will typically be picked up in childhood. A tiny vsd causes no symtoms and the murmur may be missed. If the pressure in the lungs is elevated, there may not be any net blood flow, and no murmur. However, that patient will be blue (cyanotic) due to low ox ygen and have lung disease.