How does this work?
I do have the boardgame, but how does the Vassal version work?
How do I roll the dice, how do the car(s) move and how do I draw a card and where do I put it?

You can’t create your own rally. When you choose one of the 3 pre-prepared rally stages (Etape) you get a panel with dice and cards.

To roll a die/dice, select one or more by clicking or shift-clicking, right-click Lancer (CTRL R).

To take a card drag one from a stack of cards on the panel. Click on your color car in the toolbar to open your driver panel, to have a place to drag the card to. To view the rear of the card right click it and choose Retourner (CTRL ]).

The cars are at the bottom of the panel, you simply drag one into place. You can rotate it with the left and right arrow keys.

And the green button - is that for Time Attack seconds (up or down)?

I got a 19" screen but can’t see the whole layout. Is there anything I can do to fix that?

The round marker button is to remove markes you can place on the dice.

The big green arrow button is to mark end of turn.

You can zoom out with the standard zoom controls. See the VASSAL help.