Based on your description you have built your application by using the following components:
- Your application (closed source).
- Liraries from the LGPL edition of Qt like QtCore, QtGUI, and so on (LGPL).
- Qt Virtual Keyboard plugin (GPL).
Inluding Qt's LGPL edition libraries in your closed source application is allowed by the license as long as you follow the LGPL rules, which usually implies that you link dynamically to Qt and that you allow users to upgrade or replace the LGPL parts (i.e. the Qt libraries) with their own version if they wish.
However, by including and linking the Qt Virtual Keyboard plugin with your product, you are arguably violating the GPL because you are distributing it and combining it with your program as a plugin. Even if your application uses the functionality of the GPL program only occasionally or only in response to certain user actions (e.g. when the user loads a specific file), you are still distributing the GPLed work with your application, so you must abide by the GPL terms in order to do so.
The GPL allows you to include a GPL program with a closed source program only if the GPL program and closed source program are clearly separated so as they would not be considered one program. The GPL calls this an "aggregate" in the license text. Including the Qt Virtual Keyboard plugin in your application as you have stated sounds like it is combined as to form one program, not aggregation:
When is a program and its plug-ins considered a single combined program?
If you want to maintain GPL compliance you should not distribute Qt Virtual Keyboard plugin or any other GPL library with a closed source product if your closed program makes use of any GPL programs as libraries or plugins.