In VBScript there are several subtypes for integers including integer and long. VBScript will attempt to determine what type of value you are using and use the appropriate subtype.
Integer can store a value between -32,768 to 32,767 and long can store a value between -2,147,483,648 to 2,147,483,647. That value that you are using is greater than this and will result in an overflow.
You can use the VarType function to see what type your number is interpreted as. You may use Double to represent larger values.
This answer looks interesting. Maybe you can use it as part of your function.