I've taken out this icon SVG and I've edited it in Inkscape to make the double tick icon look like the WhatsApp one.
So, later in Android Studio I put the original vector drawable icon:
<vector xmlns:android="http://schemas.android.com/apk/res/android"
android:height="24dp"
android:width="24dp"
android:viewportWidth="24"
android:viewportHeight="24">
<path android:fillColor="#000" android:pathData="M0.41,13.41L6,19L7.41,17.58L1.83,12M22.24,5.58L11.66,16.17L7.5,12L6.07,13.41L11.66,19L23.66,7M18,7L16.59,5.58L10.24,11.93L11.66,13.34L18,7Z" />
</vector>
And then I've replaced the pathData with the SVG path edited in Inkscape:
<vector xmlns:android="http://schemas.android.com/apk/res/android"
android:width="24dp"
android:height="24dp"
android:viewportWidth="24.0"
android:viewportHeight="24.0">
<path
android:fillColor="#000"
android:pathData="M 0.41,13.41 6,19 7.41,17.58 1.83,12 M 22.24,5.58 11.66,16.2 11,15.5 9.5,17 11.66,19 23.66,7 M 18,7 16.5,5.5 4.5, 17.5 6,19 Z"/>
</vector>
I can see the new icon in Android Studio preview, but when I run the app it doesn't show. Normal icons does show normally, but this one doesn't.
It's a weird behavior that I cannot understand, I'm pretty new into vector graphics.
Why is that?
I've found the answer by myself. Looking at the original icon syntax it appears that spaces should be given with an L
letter.
<vector xmlns:android="http://schemas.android.com/apk/res/android"
android:width="24dp"
android:height="24dp"
android:viewportWidth="24.0"
android:viewportHeight="24.0">
<path android:fillColor="#8A000000" android:pathData="M0.41,13.41L6,19L7.41,17.58L1.83,12M22.24,5.58L11.66,16.2L11,15.5L9.5,17L11.66,19L23.66,7M18,7L16.5,5.5L4.5,17.5L6,19Z"/>
</vector>
Now it works. Funny how I didn't know anything about vector drawable, and still found a solution.