According to a post from their Community Manger PezRadar on the forums, Blizzard has delayed the rollout of the Vessel of Hatred patch/release. This is due to a "small technical issue" that is preventing all regions from having access to the expansion at the same time. As such, until everyone can play...no one can play.

At the very least, Steam players - who did not have a pre-download time days in advance, like Battle.net or console players - can download the patch in the interim.

We'll know more "soon", but being Blizzard's brand of Soon(tm), we might need to be patient.

Edit: An update from PezRadar tells us that it'll be upwards of two hours before Vessel of Hatred's launch is good-to-go.

Edit2: It's apparently been fixed! Happy hunting!