As Vieja age they tend to get more and more aggressive, and as oscars age they seem to me, to become more and more docile.
If that turned out to be the case in your tank, the oscar would be at a distinct disadvantage, especially if your tank wasn't at least 6 ft long, preferably more.
This may also depend though, on what species Vieja you have.
Some like breidhori and synspilum can be fairly docile,
(breidhori)

while others such as zonatus, argentea, or regani can become psychotic.
(argentea)

(regani)

It would also help if both species were young when you got them, added to the tank at the same time, and were able to grow up together, each establishing a distinct and respected territory. (this is also where a long enough tank comes into play)