The Blessed symbol is silver, to which vampres are somewhat allergic. Who is to say that spiritually enhanced silver does not have an aura of its own. The symbol itself maybe of no importance at all.
As for the dust, these feral vampyres are failed transitions, nothing more than animals. That may make a difference. Though even Vyrewatch corpses need special treatment to 'bury' them.
I can only guiess about the empty graves, but they may have been hold overs from when there was a population there. More likely it was grave robbers, like the role you played in The Creature of Fenkenstrain hints at.The purpose of adventure is to shine light into dark places,
Poke monsters with a sharp stick, Then steal anything that isn't nailed down!
To the Manor Born QFC 185-186-367-65788716
Padomenessaid: What about the mysteriously empty graves in the vampyre infested haunted woods?
Along with the fact that if you are wearing a holy symbol(must have been blessed) when a vampyre in the haunted woods or mines attacks you it gives you a message?:
"The vampyre is weakened by your holy symbol of Saradomin."
They drop vampyre ashes/dust which cannot be used in a burial ceremony but only in herblore.
It's Morytania, they probably just got up and walked away. Not to mention the mad scientist bent on creating life living just north of there.
It seems like, rather than being holy/unholy, we can just think of it like Saradominist things were specially enchanted to weaken zamorakian things, and presumably vise versa. You have to remember all this was written before the two of them were given more moral grayness in the story, which is explained by excessive saradominist propaganda.
I see one issue. Vampyrium doesn't necessarily fit the description of "devoid of life"... In fact there were many other creatures there that the vampyres fed upon before they came to taste human blood in ***linor. Also Vampyres were originally introduced to social systems and civilized behavior by Zaros and as such would technically be able to invoke his blessing through prayer as any other being would on ***linor. This leads me to believe that their weakness to silver, the salve, and various other "holy" saradominist artifacts is a biological aversion rather than a spiritual version. Especially with haemalchemy leading to the creation of a way for Vyres to completely bypass the Salve's effects. Vyre corpses also release an ethereal spirit upon cremation. which makes it seem as though they have a soul as well.