Hi, welcome to the VC forum.
That sounds like a complex project. We don't personally have any experience interfacing an Arduino with a car.
Someone here might be able to help you interface VoxCommando with a functioning Arduino project.
1. Which parts of your project do you already have working?
2. Which part do you need help with?
The Arduino relay video that you watched is an extremely simple example, where no information gets passed from the Arduino to your computer.
Starting your engine and locking the doors is probably fairly similar (assuming you already know how to control your car from an Arduino).
To get status information from the car requires passing information to VoxCommando. Here are is an example demonstrating
two-way communication.