I've had a number of monitors watching different jobs; Process Monitor, Comport Toolkit and Device Monitoring Studio. DMS was the most informative of the bunch, but only reflected exactly what VBs immediate window told me anyway. Arduino failed to send an 'OK', or the laptop never saw it. Process Monitor didn't really show anything strange, other than a double Thread Exit invoked in VB around the time of failure, but I'm not entirely convinced it was significant.
Currently running another test with Serial.flush() after the Arduino's 'OK' line. So far so good...
[EDIT]
Nope, crashed after almost exactly 8hrs. Reset the Arduino and had to un/re-plug twice before VB would acknowledge it's existence again!