The medium depth palatal vault with a well-defined rugae incline provides an optimal balance between surface area for suction and stability and patient comfort. It allows the denture to achieve effective retention through peripheral seal without excessive bulk or trauma. The presence of a well-defined rugae incline enhances the mechanical interlocking and proprioceptive feedback, improving denture stability during function. In contrast, flat or shallow vaults lack adequate surface area for suction, while deep and narrow vaults can cause discomfort and compromise retention due to reduced denture base adaptation. Irregular vaults with undercuts complicate denture seating and may impair retention. Thus, a medium depth vault is most favorable for complete denture retention.

Reference: Contemporary Prosthodontics, William J. O'Brien, 5th Edition.