There are several different case laws that have established, and suggest that an individual has a constitutional right to vegan food (if you know of any others, please send them our way).
The standard for this type of constitutional protection was established in the conscientious objector case (which extended constitutional protection to moral or ethical beliefs about what is right and wrong that are sincerely held with the strength of traditional religious views), beginning with the United States V. Seeger, 380 U.S. 163 (1965) and Welsh V. United States, 390 U.S. 333 (1970). Recently, an inmate in the federal prison system sued to receive vegan meals in accordance with his ethical beliefs. After the court reviewed the facts and issued a preliminary order requiring the prison to provide the inmate with soy milk, the Bureau of Prisons added complete vegan meals to the menu at all of its facilities; Maydak V. United States, N. 97-2199 (DC Dist. May 8, 2000). Several years ago, the Equal Employment Opportunity Commission noted that the codified standard that the government used to define religious practice derived from the same legal standard listed above. In doing so, the commission ruled that a vegetarian bus driver was entitled to legal protection for his ethical beliefs and so could not be required to distribute coupons for free hamburgers; Anderson V. Orange County Transit Authority, charge No. 345960598 (1996).