Decryption isn't illegal. Bypassing encryption without a license is illegal. There's a difference.
When you play a DVD you're decrypting it using a licensed piece of hardware. Windows and Mac both have dvd playback licences (or they did, it looks like Windows now charges extra). A fair bit of software either comes with a license or technically requires you to buy one.

https://vlc-user-documentation.readthedocs.io/en/latest/support/faq/legalconcerns.html

Specifically the portion on commercial or broadcast usage.

You will need to contact the licensor on how to comply with these licenses. This goes for playing a DVD with VLC for your personal enjoyment ($2.50 one time payment to MPEG LA) as well as for using VLC for streaming a live event in MPEG-4 over the Internet.