A simple way to get Whatsapp messages into vMix is to install the Whatsapp Windows Client. This client mirrors the messages of a cell phone on the Windows desktop.

With vMix clipboard copy you can then use vMix to get Whatsapp messages that you have marked with the mouse and copied to the clipboard with CTRL-C. You can copy text from any application as long as the text can be marked.
The fade-in/out can also be done by vMix clipboard copy. (Titel rein/Titel raus)
A vMix title example is included. Title file name (whatsapp.gtzip) and text field(TEXT.Text) cannot be changed in the software.

vMix clipboard copy download here