New Allegiances

Turn  into a vampire
Vingalmo mentions an opportunity of turning a person with access to valuable information into a vampire, thus turning them into a spy for the "family".

The three candidates are:
 * Valindor in Riften
 * Cairine in Markarth
 * Haran in Winterhold

For this Vampire Seduction will need to be used, and then the target must be bitten.

Return to Vingalmo
Returning to Vingalmo may yield an item like:
 * Necklace of Minor Stamina
 * Potion of Blood ×3

Bugs

 * The candidate Valindor tends to be mentioned more than once by Vingalmo, this is a show-stopping bug, since Valindor cannot be "turned" more than once. Thus breaking the Radiant vampire quest chain. Typing  into the console, manually completes the quest (tested, seems to work). Warning: Only use console commands if you know what you are doing, and save the game before trying this!

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