If you have a two or more monitors setup and you want your videos to open upon a second or third monitor, full screen, then do the following.
Go to Tools and then Preferences or press Ctrl + P. Click on the Video icon towards the top.
Check on the Fullscreen option in the Display section.
Then next to the Fullscreen Video Device option. It's probably set to Automatic. If I wanted for VLC to automatically play on my second monitor, I would click on the word Automatic and switch it to \\.DISPLAY2
If you got more than two monitors you'll probably see them all listed there. Choose accordingly.
Then click on Save. If your settings don't seem to be holding, change your settings and then just shut down and restart VLC.