I see chakula ... vyakula is simply the plural form, which may imply several different types of food.
Other examples of ch- prefixes in the singular becoming vy- prefixes in the plural: choo - vyoo; chumba - vyumba; cheti - vyeti; cheo - vyeo; chungu - vyungu; chuma - vyuma; chombo - vyombo.
Can "vyakula" and only "vyakula" be correct? There is some interesting grammar here.
The difference between chakula and vyakula is strange, probably because in many languages the word for "food" is plurale tantum.