Ok, so i know that a Vieja isn't a flowerhorn, but. . . with a flowerhorn there is a way to sex them because quite frankly, looking at them isn't like looking at jack dempseys. This link shows how to look at the vents on a flowerhorn to see what a male and female vents looks like, and it may not give you a concrete answer, but it may steer you the right way. Just be ready with a net because you'll need to handle your fish in a way you may not like haha.
Even if it doesn't give you an answer, you may like the new knowledge.