If VB closes the port and the Arduino sends serial data, TX WILL NOT FLASH. VB continues to randomly close the port, say the device connected is not functioning, or just tell me outright that the port doesn't even exist (even though the device manager may show everything's working fine).
I know that it might take a while, but what happens if you have the Serial Monitor application monitor the serial port, instead of the VB app? Does the PC continue to receive data (or for longer than the VB app does)?