This function will build a C-String similar to printf. It is server-side only!
int iCreepers = 3;
char * pFortress = "My Awesome Fort.";
char * pOutput = UTIL_VarArgs( "There are %i creepers outside of your %s.", iCreepers, pFortress );
// prints: There are 3 creepers outside of your My Awesome Fortress.
This is useful for quickly building a composite string.
char * pOutput = UTIL_VarArgs( "X:%f Y:%f Z:%f\n", vec.x, vec.y, vec.z );