There has been a shake up with the Discovery Channel and content rights late last year. I believe it kicked in on Feb 1 (i.e. now). As you say ESPN is in the VIBE Pack (not sure where it was before) so you'll need $6 to subscribe to that package to keep watching it.
There's also a new Eurosports channel for another $8 or so a month you can try out now for free.
ESPN is still part of the Vibe pack, not Variety pack. It's definitely shown every day 24/7.
What does it say Channel 150 is in your TV Guide?
You said have Optus Sport and Knowledge subscriptions which don't include ESPN... perhaps you had the Vibe pack for free last month?