The Vyntadi Expanse was a region of space in the Delta Quadrant.
In 2374, Harry Kim and Lyndsay Ballard undertook an away mission to a class M planet in the Vyntadi Expanse, where they had detected dilithium. However, the sensor readings had been falsified by a Hirogen hunting party, and Ballard was fatally wounded in their subsequent attack. (VOY: "Ashes to Ashes")
This region was only mentioned in dialogue.