For what it’s worth, I may have found out how to mute vibrations from an app using the adb command and removing VIBRATE from the app manifest through this command (while in a adb shell on the devices):
appops set [PACKAGENAME] VIBRATE ignore
Unfortunately I’m not sure what packagename Signal uses, no package on my device is named anything containing the string signal.
Get the list of installed packages from the device, while on your computer (with the device connected via adb):
adb shell cmd package list package
Search for a specific string like this:
adb shell cmd package list package | grep "specific-string"
The Signal packagename is: org.thoughtcrime.securesms.
So to stop the vibrations, you should use this:
appops set org.thoughtcrime.securesms VIBRATE ignore
Personally it continued for me, but I force killed the app and reopened it.
Haven’t received any vibrations afterwards. But I’m not 100% sure it did the trick yet.