A Vieja community in a 400gal would be awesome, they are some of my favorite fish and I have always wanted to do a huge community of them.
However, going back to the initial issue, I would not trust the vieja to not bother the geophagus. Your issue with the Oscars was that they were being conflictual with the geophagus: Vieja are definitely a worse choice if you thought your Oscars were too aggressive. If you remove the geophagus, then by all means, go for it. But if your looking for fish that will be peaceful and leave your geos alone, I definitely would not recommend a bunch of Viejas.