Without seeing the VB code, nobody can say for sure. One thing might be that you send e.g. "1\n" in which case '1' will switch the led on and '\n' will switch it off again.
You can test your Arduino code with the Serial Monitor; once that works, you can move to the VB side of things.
I suggest that you have a look / try the approaches from the link in post #2. It gives ideas for the receiving of data by the Arduino; next you can write the matching functionality in VB.