Can’t get VGA output to work with MacBookPro5,3 OS X 10.6.8

I have a ~2009 MacBook Pro (model MacBookPro5,3) with OS X 10.6.8.

I'm trying to use a Apple Mini DisplayPort to VGA adapter to connect to an external monitor. I've tried two different monitors and two different VGA cables so far, and no luck. Both work fine with my ThinkPad; simply plug and (dis)play.

I believe I've already tried every permutation of the order of connecting things, as well as with power on or off. But the MBP just doesn't seem to recognize the monitors.

For example with monitor powered on, when I insert the DisplayPort adapter, the MBP screen turns blue for a second or two … but then I don't see anything on the monitor or anything change in System Preferences > Display.

I've never tried to connect an external monitor to a MBP before — I didn't expect it to be this difficult. Is there something dumb/obvious I'm overlooking??

EDIT: Showing System Profiler result for Graphics/Displays when the external monitor is connected but not recognized via the mDP. Note that it shows two adapters:

1.

NVIDIA GeForce 9400M:

  Chipset Model:    NVIDIA GeForce 9400M
  Type: GPU
  Bus:  PCI
  VRAM (Total): 256 MB
  Vendor:   NVIDIA (0x10de)
  Device ID:    0x0863
  Revision ID:  0x00b1
  ROM Revision: 3448
  gMux Version: 1.8.8
  Displays:
Color LCD:
  Resolution:   1440 x 900
  Pixel Depth:  32-Bit Color (ARGB8888)
  Main Display: Yes
  Mirror:   Off
  Online:   Yes
  Built-In: Yes
Display Connector:
  Status:   No Display Connected

2.

NVIDIA GeForce 9600M GT:

  Chipset Model:    NVIDIA GeForce 9600M GT
  Type: GPU
  Bus:  PCIe
  PCIe Lane Width:  x16
  VRAM (Total): 256 MB
  Vendor:   NVIDIA (0x10de)
  Device ID:    0x0647
  Revision ID:  0x00a1
  ROM Revision: 3448
  gMux Version: 1.8.8
  Displays:
Display Connector:
  Status:   No Display Connected
Display Connector:
  Status:   No Display Connected