Do you get the same problem when using the Arduino Serial Monitor, or only with VB?
It sounds like the Arduino is resetting, and a common cause of that is someone trying to power a motor from the Arduino board. Motors generally draw too much current and should have their own power supply.
Does the program seem to work properly if you disconnect the motor?
If the problem exists with both the Serial Monitor and VB then you should sort out the problem with the Serial monitor before you introduce the complication of VB.
...R