There are many other vocabulary training applications out there than just Vocup.
Vokabel Trainer 5 was last updated in 2013 but has much more features than Vocup. Using Vokabel Trainer 5's vocabulary books in Vocup may be useful as well as implementing some more features.
Vokabel has native apps for UWP and Android. It offers many different ways to practice vocabulary words and has a very primitive file format. Vokabel has many interesting features for inspiration and would be a candidate for interoperability.
There are many other vocabulary training applications out there than just Vocup.