This is a game where "detect evil" exists, but it's not like evil characters have it tattooed on their foreheads. It's a challenge to play well, and a lot of people treat "evil" (and CN, for that matter) as "I'm going to act crazy because I'm a PC and can get away with it." If your players are up to the challenge, and there aren't any codes to get in the way, there's no reason good and evil characters can't hang out.
Evil doesn't have to mean cartoonish villainy. It comes down to whether or not your players can see that.
You can probably pick and choose the things that worked alright out of the 3.0 Epic Level Handbook, but having played some crazy epic stuff before, I treat level 20 as an "end point," and aim to complete everything by then.
The way they wrote it makes it tricky. If it were me, I'd probably make vows monk only, but if a player was insistent and had a compelling reason for wanting to take a vow, at the very least it would cost a discovery, ninja trick, or something similar.
It'll be one more thing you'll have to do, but you might consider making any NPCs with class levels Gestalt as well. If the low level fighter types can suddenly heal and buff themselves because they have cleric casting, the encounters should even out a bit.