[RPG] Does anyone have any Epic Stats for Arch Lich Vecna

The 3.5e Dungeon Master's Guide has some rules for firearms in the Building a Different World section (p144).

Page 145 provides details for Renaissance weapons: pistol, musket, bomb and smokebomb. It also provides some details and considerations for how gunpowder and firearms work. They're stronger or as strong as a heavy crossbow, but have some different drawbacks.

In addition, the following page details Modern and Futuristic weaponry (from revolvers to antimatter rifles). The game is much less prepared for how powerful these are, though! You may be able to use the futuristic weapons if you reskin them as powerful magic weapons.

Bear in mind this is all a starting point. The chapter containing these rules is provided as a sort of springboard rather than a complete ruleset.

No, there is not.

Generally, “checks” (ability checks, skill checks, initiative checks) do not have critical effects, and there are some very good reasons for that (5% failure rate is inappropriate for most of them, plus in many cases what would happen on “auto-success” is very unclear: take Jump for example).

Natural-1s on saving throws, attack rolls, some of the opposed rolls (grappling, etc), is auto-failure. Natural-20s on the same rolls, and so on, is auto-success. In attacks’ case, there is also a chance of a critical hit on a natural-20 (or less for some weapons).

Generally speaking, expanding these rules to other sorts of checks is very ill-advised.