@freemo fun experiment! It is not considered a strong acid in that it does not fully dissociate in water, but it still, IMO, deserves a little cautious respect. Fully protonated phosphoric acid (H3PO4) is ~150x stronger than acetic acid (vinegar). In your mixture, it may have been present to pronate the vitamin to make it positively charged and therefore more soluble. The second deprotonation (of H2PO4-) is ~300 times weaker than acetic acid.