I'm pretty sure this advice won't be of huge help, but as long as I've been vibrating mantras and runes I have been using a modified version of the technique I use for my vocals in my band (keep in mind these are UNCLEAN vocals techniques I modified not normal singing) I have gotten to a point where I do vocals in a punk rock / metal band so I sing both normally as much as I spend time growling my lyrics.
The part of this story that can be of help, try to while vibrating, slip a bit of a growling/raw tone into the mantras or runes. After listening to HPS Maxine's SATANAS vibration audio for the RtRs I started to 'tremble' my lower jaw during the vibrations, this added even more power to my vibrations.
But in short, when vibrating, try to force your voice into a rawer tone, towards a VERY SLIGHT growling tone.
This was instinctive for me since I've been singing metal often the past 6 years, I just had to make the right adaptions.
HAIL SATAN
HAIL GAMORI
HAIL ALL THE GODS OF DU'AT
