I totally get what you mean! It's frustrating when Android keeps asking you to use a certain app to open a file type, even after you've chosen a different app.
To fix this, try going to Settings > Apps > [Three-dot menu] > Default apps > Show system > Vanadium, and then tap "Clear defaults". This should remove Vanadium as the default app for .txt files.
Alternatively, you can also try going to Settings > Apps > [Three-dot menu] > Default apps > Opening links, and then toggle off "Vanadium" for .txt files.