For those who don't know, DBM is a fan-made on-going comic based on pre-GT Dragon Ball Z. The basic story is that the Kai's have organized a massive tournament of the strongest fighters of 20 of the many parallel universes for the prize of three wishes from Namek dragon balls.
So, naturally some of the canon DBZ characters decide to compete in the big fight (namely Trunks, Goten, Pan, Goku, and Vegeta), and we see early on that the other contestants from parallel universes are all from realities of alternate endings.
WHEW! Okay, so the TG of Vegeta is specifically an off-screen thing. Now, this is a Vegeta from a universe where Kakarot never became 'Goku' but instead slaughtered everyone on Earth and ended up joining Radditz, Napa, and Vegeta to eventually overthrow Frieza and wreak havoc in their universe.
During the fight, Vegeta goes up against a magician-girl of sorts (not sure if she's based on a canon character) who has the power to swap genders, and thus transforms the evil Vegeta into...uh...Vegeta-ko.
Here's the manga on g.e-ihentai.org (Be sure to download NoScript before you go onto the site):
http://g.e-hentai.org/g/439431/09f2ffa0d0/
And here's the pics.