I was able to modify the apk in order to send the "GT-I9300" string, so that I didn't have to modify my build.prop. Those that don't wish to modify build.prop, or can't (because you aren't rooted) can give it a try and see if it does the trick for you.

http://www.mediafire.com/?fp04n29phdzeram

Of course, thanks to the OP, as I wouldn't have known what to modify without the information about the build.prop changes.

For the purpose of disclosure on what was modified, and for those that wish to make the changes manually, you can use apktool to decode the apk, then edit smali/com/vlingo/sdk/internal/deviceinfo/PhoneInfo.smali and modify:
"method public getModel()" and "method public getDeviceID()". Right before the return at the bottom of each function, put: const-string v1, "GT-I9300" (replace v1 with the value the return uses, v0 for getModel, v1 for getDeviceID.