In an adult, if a small vsd is causing a murmur, does that mean its significant enough to be seen on echo? if you can hear it, echo will pick it up?

Yes: If the murmur is a vsd it would show up on echo.Echo is so sensitive it would pick up a vsd or other defect even if it were not heard.There are many vibratory murmurs that are benign, with no defect involved.

Yes: Yes. The echo is usually very good at visualizing small vsd's.

Yes: An echocardiogram performed in a center with expertise in congenital heart disease will easily show a ventricular septal defect.