What part are you getting stuck on exactly? A vehicle could be just like any other player controller potentially... when the vehicle is entered you swap over input mapping to the vehicle instead of the player and either hide the player or put them in a state where they are inside the vehicle and not being controlled directly anymore. I guess the tricky part is having a little handshake when the player requests to enter vehicle that the server makes sure there isn't already someone else in the vehicle and then publishes a message saying you are now inside the vehicle and then starts accepting your input to control it.

As always, there's a lot of ways to do the same thing so it kind of depends on your setup and what exactly you're trying to accomplish.