Beside venting.

It depends on which Vieja you are trying to determine the gender of.
Some are fairly easy determine, some are almost impossible just by looking at outward appearance.
The shape of the profile is often a clue, but......if your Vieja are young, it usually doesn't work.
I have kept and bred a number of Vieja.
One was V. bifasciata, the profiles of males are steeper than females, but really only visible when you see males and females together.
I have also had V. zonata spawning my tanks, but could only see differences after they were about 2 years old.
For me....V breidhori are the easiest.
1st...a male V. breidhori

now a female

note the females more streamlined head profile. and the dark blotch in the dorsal fin, and her darker overall colors.
These are the indicators I look for,but in other Vieja species only show up when in spawning mode.