[RPG] Are there official rules for Vecna published for 5E

Official Stats are in Volo's Guide to Monsters

Volo's Guide to Monsters (released November 2016) includes many additional Illithid variants and related creatures. One of these is the Elder Brain.

The official Elder Brain description can be found on pages 173 and 174 of Volo's Guide and includes lair actions, regional effects, a full stat block, and lore on the nature of the Elder Brain.

But Matt Mercer's Elder Brain is homebrewed

Matt Mercer's Elder Brain appeared well before the publication of Volo's Guide, and thus is distinctly homebrewed. Additionally, Matt himself has stated:

...and at full strength [the Elder Brain] is a CR 20 creature.

This doesn't line up at all with the official publication, which only has a CR of 14. A quick search didn't turn up any exact stats for his Elder Brain, only a meager collection from the third party critical role stats site which has the following information:

• Elder Brain First Appearance: 10 K’Varn Revealed
• Armor Class 18 (at weakest), 20 (before party flees)
• 320 damage taken and “not on death’s door yet”

Storm King's Thunder uses a modified goat statblock for sheep.

D&D Beyond includes a sheep statblock, for which it cites p. 142 of Storm King's Thunder as the source. Looking at that page of the adventure, Grudd Haug area 9 (Stockyard) says to use a modified version of the goat statblock (basic rules, p. 138; SRD, p. 381) to represent sheep:

Use the goat for the sheep, with the following changes:

• It is a Small beast with 3 (1d6) hit points.
• Its speed is 30 feet.
• It lacks the Charge feature and any effective attacks.
• Its challenge rating is 0 (0 XP).

Note that the goat statblock is already CR 0, but is normally worth 10 XP - presumably because it does actually have a ram attack and a Charge trait (ineffective though they may be). The sheep statblock no longer has this attack or trait, and its hit die is also slightly smaller so it has 1 less max HP on average. Per the "Experience Points by Challenge Rating" table, CR 0 creatures can be worth either 0 or 10 XP (it's the only challenge rating for which more than one value is listed in the table).

That said, the Wild Magic Surge table (PHB, p. 104) obviously wasn't written to reference an adventure that wasn't even out yet at the time the Player's Handbook was written (one which was released a few months after you posted your question). As nitsua60's answer says, the book itself acknowledges that it can't include statistics for every animal in the world, and leave it open for the DM to approximate something reasonable based on what's provided. Even without owning Storm King's Thunder, one could reasonably have decided to use the goat statblock (as-is or modified) to represent a sheep if needed - SKT simply explicitly codifies that advice for use in the adventure.