I have an old broken viewsonic VG150 LCD monitor. Basically, it cannot be
turned on even though the adapter is supplying a healthy 12V DC (measured
with load).

I opened the case and measured a pin labeled +12V and found only 2V when the
power switch is held down, and goes to 0V when released.

The front panel power switch itself is functional -- I short out the two
contact points with a wire and got the same 2V as above.

This could mean the circuit that controls the main power (probably a power
transistor) is bad, or there is a short circuit dragging down the voltage.
Since there is no smoke when I held down the switch, I'm hopeful it's the
first case.

The power supply (external adapter) is 12V 4A. The wires leading to the
front panel power switch is very thin, and the switch itself is a push
buttom with momentary contact. So I figure the power switch is not what's
directly turning on the power; it must be controlling a semiconductor that
turns on the full 12V 4A to the entire unit. However, I did not find any
power semi-conductor that looks like it can switch on/off 4A of current.
Here's a photo of the circuit board:

http://www.smugmug.com/gallery/6159236_bYvW4/1/388058044_TTcnS
(put mouse on the photo and click "original" to see full resolution)

Any idea which chip/transistor controls the 12V 4A to the rest of the
system?

To further debug the problem, I need a circuit diagram. Any idea where/how
to get a circuit diagram of a viewsonic monitor? I looked on viewsonic's web
site and the phase "cicruit diagram" or "repair manual" returned zero hit.

Without circuit diagram, the only thing I can think of is to solder a wire
directly from the input 12V DC to one of the +12V lead. If this works, it
means the unit cannot be turn on/off via the front panel switch. If it
doesn't work, it may generate some smoke...