As between a ventral hernia and an incisional hernia?

The same. A ventrlal hernia is a hole anywhere along the front or ventral surface of the abdominal wall. An incisional hernia is a type of ventral hernia that forms after the abdominal wall is weakened by prior surgery.
See below. Sometimes these terms are used interchangeably. Ventral hernia usually refers to a hernia that occurs in the anterior abdominal wall. These may be primary (occurring without prior surgery) or secondary to an operative incision. Incisional hernias are just that - hernias that occur in an old operative incision. These are usually in the anterior abdominal wall as well.
Different. Ventral means front and can be diastasis , umbilical or otherwise without prior surgery! incisional is in an area of an incision meaning prior surgery.