How do I set custom ringtone for video calls (e.g., WhatsApp video calls) from a particular contact

I am trying to set all ringtones (including for audio and video calls, both on the phone itself and over WhatsApp) for one particular contact to silent ringtones. I have made a silent (well, not fully silent - it looks like it insists on adding a vibration) ringtone and managed to have it show up in my list of ringtones in the phone settings and to set it as a ringtone for the desired contact. This seems to take care of the audio calls (including via WhatsApp) but video calls still go through with the default ringtone (not at all silent). How do I fix this?

Also: is there a way to have a fully silent ringtone (no vibration either)?

I should point out that I am typing this question here after **many** wasted hours on both mine and my husband's parts searching the web and making multiple tries to nearly no avail (the best we could do is what I described above to be the current situation). Yes, I am afraid the level of frustration has gone through the roof, as this is such a simple request to have (on a non-smart phone, it literally takes a few seconds to do), I find it quite unbelievable that this option is not implemented on iphones as one of the most common and accessible settings one can have. Does anyone know how to make a request to iOs developers?

Thank you to anyone who can help!