In an adult, is an echocardiogram likely to miss a small vsd (small, but causing a murmur)? or is it reliable to rule it out? usually found as child?

Depends: If the indication for the echo is "rule out vsd" - the echo is very unlikely to miss it. A routine echo done for other reasons could miss a very small vsd. (btw, a very small vsd doesn't require treatment).

Unlikely: If we can hear the murmur with a stethoscope, then an echo probe will usually be able to show the blood crossing the vsd, even if the 2-d imaging doesn't clearly show the defect. However, if the echo windows are very poor (e.g., with obesity, chest wall deformities or fused ribs, lung disease like emphysema, etc.), then it may be hard to visualize the heart at all, and a vsd could then be missed.