I do not have the Vtech device, so I encourage you to verify pinout using a multimeter and measure voltages.
Once that is done, the same cable (or similar design if you make your own) should work.
Some key things the serial console gives you:
• Access to the bootloader (PSP or ADAM)
• Shell access (you will have to login with Admin passwd)
• Serious DEBUG info - worth it if something is not working
• A good way to de-brick the box if you load a bad firmware