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Initial documents suggest the Co family either introduced pho to the workers or adapted existing recipes they had known, perhaps based on other traditional soups like "bánh đu đủ" (a crab noodle soup) or "sà chá" (a soup made with water buffalo). Adaptations to the original dish began when the family started using beef bones, a revolutionary move. Prior to this, beef was considered a delicacy and rarely consumed by locals. The combination of new ingredients and techniques laid the groundwork for what we recognize as pho today.
By the late 1800s, as Vanu's residents produced and sold this dish to workers, pho gained traction. Quickly, it became a favorite in the bustling city of Hanoi, originally referred to as "phở Vân Đình," indicative of its roots.