Beyond its ability to prevent scurvy, vitamin C has no further role in infection prevention. Once you have enough vitamin C in your system to perform its job (acting as a cofactor for collagen synthesis and the production of certain neurotransmitters), the excess serves no purpose and gets excreted directly into the urine.
Interestingly, guinea pigs were used in medical research mainly because like us, they can't store excess vitamin C and need a continual supply in their diet.
I have no idea what's actually in "Airborne", but given that they use clinically meaningless weasel-wording in their advertisements like "provides immune support" makes me think they're trying to skirt FDA regulation.
EDIT: Wow, I just realized this side train of discussion comes from February of 2020...