As I mentioned earlier, I had most success from Device Monitoring Studio (most of the other monitor programs would just say "port is busy" and not even attempt to connect. The DMS output mirrored exactly what the VB IDE was reporting, byte for byte.
The more I search for info on VB's serial port, the more I find a trail of irate developers who swear that the VB serial implementation is rotten to the core (at least pre-Net Framework 4.0). I have stripped the SerialDataReceivedEventHandler out as I'm sure it's overkill, I already know when a command is coming from the Arduino, I don't need a thread to watch it.
I'm currently running a skeletal VB app that just prints command strings to the Arduino. Even this froze in a debug build! Once the release build has failed later today I will be compiling in 2010 with Net Framework 4.0.