Oblivion:Vampire Dust

You can collect Vampire Dust from the corpses of vampires that you have killed.  It is a valuable Alchemy ingredient.  Vampire dust is even more valuable if you complete The Order of the Virtuous Blood quest, which enables you to sell vampire dust for 250 gold.

However, vampire dust can be a bit tricky, because there are actually two different substances, both of which appear identical and are identified as 'Vampire Dust'. You will primarily notice this if you are carrying both types: in your inventory you will see two different stacks of vampire dust; in the Alchemy menu you will be able to make some interesting potions.  There is even a third type of vampire dust, but at least the third type is given its own name, 'Ashes of Hindaril'. 

Standard Vampire Dust
Standard vampire dust (construction set ID VampireDust, FormID 00009182) is found on all the vampires you kill except:
the vampires in Bloodcrust Cavern during the Mages Guild quest Information at a Price.
the vampire Hindaril in Redwater Slough.
the Pale Lady
Vampire Dungeons provides a complete list of all the places in Tamriel where vampires can be found.  Vampires can be found in these dungeons at all levels, and in most cases (red circles on map), vampires are also guaranteed to be present.

Vampire Dust cannot be purchased from merchants, nor are there any guaranteed pre-harvested samples.  The only other way to find it is randomly in some Necromancer and Vampire loot chests (starting at level 9).

If you are suspended from the Mages Guild for murdering a fellow guild member, you will need to collect twenty samples of Vampire Dust (as well as twenty Daedra Hearts) to be reinstated.

Standard vampire dust can be duplicated in the Archmage's Enchanted Chest; you will end up with 11 samples of standard vampire dust.  All 11 can be sold to the Order of the Virtuous Blood for 250 gold each.  If you are the archmage and strapped for cash, this is an easy way to make a lot of money.

In most cases, it is necessary to kill the vampire before Vampire Dust appears on the corpse.  However, the Afflicted Brethren found in Gutted Mine as part of Azura's quest always have Vampire Dust and can theoretically be pickpocketed (although to complete the quest you still need to kill them).  No additional Vampire Dust appears if you pickpocket one of the Afflicted Brethren and then kill him.

thumb|left|500 px|Locations of Vampires

Rare Vampire Dust
Rare vampire dust (construction set ID MG13VampireDust, FormID 0004872C) is found only on the vampires in Bloodcrust Cavern during the Mages Guild quest Information at a Price.  The Vampire Dust is always present on the Vampire and can theoretically be pickpocketed. Its effects and appearance are identical to regular vampire dust; the only noticeable difference is that its weight is 0.1 instead of 0.2.

Rare vampire dust cannot be sold for the 250 gold bounty to The Order of the Virtuous Blood.  Similarly, it cannot be used to pay the toll in the Mages Guild Suspension quest.

If you leave rare vampire dust in the Archmage's Enchanted Chest, the rare dust will be changed into regular dust: you will end up with 11 samples of regular vampire dust.

Ashes of Hindaril
The Ashes of Hindaril (FormID  ) are the third type of vampire dust, and are only found on the corpse of a single vampire, Hindaril, who lives in Redwater Slough.  You must kill Hindaril before the ashes will appear.

Technically, these ashes have the same alchemical properties as standard Vampire Dust. However, since they are quest-specific (for the quest Vampire Cure), they cannot be used for alchemy, sold, or otherwise removed from your inventory.  The only way to get rid of them is to complete the Vampire Cure quest, during which Melisande will take them from you.  Even if you could somehow put them into the Arch Mage's Enchanted Chest, these ashes would not be duplicated.