Vip3Aa (vegetative insecticidal protein) secreted by Bacillus thuringiensis (Bt) is highly toxic to lepidopteran insects. The Bt isolate M190 produces Vip3Aa35 at high concentrations, and Vip3Aa35 was found to be very effective against Spodoptera exigua. Unfortunately, the use of Vip3Aa35 in pest control is limited by its short shelf life when stored at high temperatures, retaining activity for only 1 month at 37 °C. To find a more stable alternative, we screened 500 isolates of Bt collected from various locations in Thailand and discovered Bt isolate 294 which produced large amounts of Vip3Aa64 that exhibited high toxicity against S. exigua but could be stored at 37 °C for up to 3 months. Vip3Aa35 and Vip3Aa64 have only nine amino acid differences between them, with six of those residues being located at the C terminus. Vip3Aa35 and Vip3Aa64 chimeras revealed that the C-terminal sequence is important for the retained larvicidal activity observed with Vip3Aa64. Various single amino acid substitutions were created to identify the key amino acids responsible for this stability. A single residue, Tyr776, was found to be solely responsible, with the Vip3Aa35:N776Y acquiring thermostability similar to Vip3Aa64 while the Vip3Aa64:Y776N exhibited Vip3Aa35-like thermostability.