Strange problem with external monitor

Hi guys, I bought a mini displayport to VGA cable to connect my 13" MBP 2010 to an external monitor (21" acer)...

I just found out that if I use the "mirroring" mode, and I set the resolution to 1680x1050 and 60hz(my monitor max resolution/hz), it's all kinda blurry...

But if I put in sleep mode ma MPB and wake it up "switching on" my bluetooth keyboard, the monitor in my MBP doesn't even start and it just use the external monitor.
In this case, there's no more blurriness but the settings are the same!
Then, I can even turn on my MBP monitor (pressing some random key on its keyboard) and nothing change, still no blurriness...

How can this happen?

By the way, there is another funny issue: if I use this "trick", the applications that should run in "fullscreen" (not really fullscreen, just the window that use all the screen size), like iTunes, don't do it anymore....
How is that?

Thank you in advance, Paolo.

 

All right, thank you...
But is there a simpler way to do it? To use the external monitor as primary one?
Because if I turn the mirroring off, on my external monitor there is just a wallpaper (the milky way one, or sth like that) and a window with a list of resolutions, but nothing else...No dockbar, not bar at the top etc..