The term 'vault part' can be broken down into two crossword solutions: ARCH (4 letters) and RIB (3 letters). An ARCH is a curved structure that spans an opening, usually made of stone or brick. It is commonly used in architecture to support the weight of a roof or bridge. A RIB is a curved beam that supports the sides of a vault or ARCH. It is usually made of wood or metal and is used to provide structural support. Together, an ARCH and RIB form a vault part, which is an essential component of many architectural structures.