What is a small one-masted vessel with mainsail and jib?

a dinghy would be a small (6 to 16 foot) sailboat, these can have almost any sail rig. the type of rig you described is called a sloop. a sloop is a sailboat with a triangular mainsail and a jib (this sail configuration is called a marconi rig, or bermudian rig), and are very common. so a small, one masted vessel with a mainsail and jib would just be called a sloop, or dinghy.