Among many things, in some embodiments, dual-function additives that enhance fluid loss control and the stability of viscoelastic surfactant fluids, and their associated methods of use in subterranean formations, are provided. In one embodiment, the methods comprise: providing a viscoelastic surfactant fluid that comprises an aqueous base fluid, a viscoelastic surfactant, and a dual-function additive that comprises a soap component; and introducing the viscoelastic surfactant fluid into at least a portion of a subterranean formation.