[8FS_Bulau]

1)...compromise with 1680 x1050. BUT I've read somewhere that this mean a double work for the Graph Card that need to re-work the game displaying
2) Vertical sync on or off for smooth the playability?

I think the idea that a lower res than the native means "double work" for video card is nonsense. A lower res is less work for the video card, and you will get smoother play and higher frame rates.
I run v-sync OFF, because I find the higher frame rates gives me smoother play, especially when panning the view, but the trade off is you get some tearing of the image in the cockpit. If the tearing does not bother you, then leave v-sync OFF.