Yeasts are part of the group fungii. They were originally considered to be plants even though they do not produce chlorophyll, now they have their own kingdom. Microbial assays for vitamin B12 are no longer considered reliable due to problems with the cross-reactivity of corrinoids. The package lists gelatin, but it is not animal derived. Unfortunately there is no simple answer to this.
Many Companies allow individual stores to decide part of their menu, or the ingredients used.
Asking an employee may not be enough. If the food in question contains an undesired element, ask if it can be substituted for, or deleted altogether. Fill out a comment card, if you think their menu does not have enough selection. If the company receives enough of these, they may decide to follow up on them. Taco-Bell do not use lard anymore in their cooking.