db.define('vehicle', {

    ...

}, {
    hooks: {
        beforeUpdate: function(values, cb){

            // Want to get my user in here.

        }
    }
});

...
beforeUpdate: function(vehicle,cb){
  vehicle.getUser()
  .then(function(user){
    console.log(user)
  })
}

beforeUpdate: function(vehicle,cb){
  User.find(vehicle.uid)
  .then(function(user){
    console.log(user)
  })
}