As I understand '\n' is interpreted as a line feed character. In VB.net vbLf is a line feed character. I have used the following code in the VB app to send the string:
ArduinoConnection.Write(Message + vbLF)
So, it seems logical to me that this would send "abc" plus the line feed. When the Arduino code encountered a line feed, it is supposed to add a null character, but even with the line SerialMessage[SerialMessage_char] = '\0'; commented-out, the string is still not evaluating correctly.