Just to make sure, you have 4GB of RAM, and 1GB is being used by the video card which make it work with 3GB?
Your OS is 32 or 64 bits? Seem to me that it is 32 bits which is terrible and it is not using your processor to its fullest neither your memory, in case you have 4GB.
The video card is not really that good, but since this game is not optimized, it can behave differently even if others have the same GPU.
Make sure you have installed the required components for the game (DirectX and Visual C++ Redist).
And about your OS, Vista is really old and in some cases can be unstable after a few updates (drivers and software).
Bellow is the game requirements (minimal on left required on right and some descriptions are in portuguese but the parts name you can pretty much figure what is what):

If you have tried any tweak you found searching about make the game run smoothly, i suggest you to not try for now, you can try and delete the file user.xml in the game folder and try run it again to see if it goes, but it doesn’t seem to be that.
Update drivers and windows itself.
And last, a new machine is in place.