Brahma's vigor is nice and all until you realise it's actually hard to abuse it cause it lasts only 3t, and it only comes after she bursts. So first of all, you'll have to invest about... 3 turns of GDATA at best to get her to burst, then burst and get vigor and get only 1 turn of GDATA to benefit out of it. If you choose to wait to use GDATA until after she bursts then your team becomes kind of a slowpoke, and you have to consider the fact that you're pretty much only going to get vigor's use in... 3 turns of GDATA and not during a burst unless you use Shingen MEX2, which you probably won't if you need to keep debuffs up with sniper shot. You have space to play around considering Raiko, but when you add it up Brahma's moveset seems to flow much better on paper compared to in practice. Her nuke also basically can't benefit from having vigor because cap. However, that's also why I recommended you drop Mammon for high debuff res bosses, because then you can run Brahma and have Shingen run MEX2.
Marduk on the other hand, while not having GDATA compensates with... basically a couple o buffs in fortress: 20% echo dmg on normal atk, extra 20% echo dmg on burst specifically for herself, and a 2.5k barrier to all. And a 400 HP/t regen on the same skill. And instant burst + inspiration to boot, and that sweet atk break you otherwise wouldn't get with Brahma that helps in some cases (GO being the most notable for you so far I'd wager). And fortress, unlike vigor, buffs dmg as long as it exists, which is a plus considering vigor's blargh scaling.