This answer was correct until June 2022, when stats for Vecna pre-ascension were released on D&D Beyond.
While D&D third and fourth edition provided statistics for Vecna, there aren't any in fifth edition (yet, at least). Matt Mercer's statistics for the deity appear to be his own invention. Vecna's entry at the Critical Role wiki has many statistics missing, as Mercer never revealed them and they do not appear in any sourcebook.
Based only on what we know about Vecna from the D&D core rulebooks, he's a lich-god capable of crafting artifacts (Dungeon Master's Guide p.208,222). This suggests his statistics are at least that of the lich (Monster Manual p. 202), a challenge rating 21 creature capable of casting 9th level spells. That's a good starting point for Vecna's statistics.
The Dungeon Master's Guide p.11 defines the power of deities. Unlike earlier editions of D&D, there's no specific definition of deities' power levels here. The lesser deity Tiamat is challenge rating 30, so Vecna's 5e statistics may be around this power level. The empyrean is a challenge rating 23 titan, with titans being among the weakest of deities. The highest challenge rating creature in this edition is CR 30, and anything above CR 20 are intended to test player skill. (Monster Manual p.9).
Vecna's statistics in 5e would thus depend entirely on how challenging you intend him to be in your campaign.
- At minimum, he's a CR21 base lich, which would be a reasonable fight for high level PCs. This might represent Vecna before his ascension to divinity.
- If you want him to be challenging even to level 20 PCs, give him stronger abilities.
- If you want him to be of truly godlike power, make him a challenge rating 30 creature.
- Anything more powerful than challenge rating 30 basically isn't something that player characters will fight.