1/2/3 etc. for weapons. and Enter button to Enter vehicle and use Scroll/space to choose from specifik mount options. Either way switching/using weapons/bombs etc. should absolutely not be mixed with the mount/dismount/use item on just one scroll menu!
This slows down the pace of the game and interaction, this ofcourse not being a brainless shooter like CoD or BF but it doesn't mean it has to be overly complex for no reason!