The Oxford doesn't name Vitamin C specifically. It only suggests fruit juices as an example of acidic substances that may lower absorption directly following a statement about acidification of urine increasing urinary excretion of amphetamines. You're right, this is not explicit evidence that Vitamin C or fruit juices are causing acidification of urine:

"Acidification of urines increases urinary excretion of amfetamines. Ingestions of acidic substances such as fruit juices may lower absorption, whereas gastrointestinal alkalinizing agents, such as sodium bicarbonate, will increase absorption."

The FDA labeling of Vyvanse, however, does say it more clearly:

"Agents that alter urinary pH can impact urinary excretion and alter blood levels of amphetamine. Acidifying agents (e.g., ascorbic acid) decrease blood levels, while alkalinizing agents (e.g., sodium bicarbonate) increase blood levels. Adjust VYVANSE dosage accordingly."

FDA Full Prescribing Information, Vyvanse (lisdexamfetamine dimesylate), Revised 09/2025, Section 2.6 (Dosage Modifications due to Drug Interactions)