Vile Death for Intelligent Undead?

[The Viscount]

Vile Death, as per the spell compendium, is for whatever reason limited to a miniscule number of undead, every corporeal undead in MMI except for the lich. It applies the fiendish template, though we can assume specific trumps general means it works despite undead not being a valid target for fiendish.
The spell mentions that the resulting creature is independent of you and can act on its own, implying that it's the fiend that's in control, since the spell works even on mindless undead.

So, the question arises. What happens when an intelligent undead receives a vile death spell? Which mind is then in charge? What would happen if a PC were to receive this spell? There's obviously no textual answer, so I want to know what the playground thinks. Invoke the possession rules, perhaps?

[Toy Killer]

If I were DMing, I would invoke the Possession rules, I may also talk to the player about taking levels in Warlock to show the fiend's power leaking through.

[Thurbane]

I think you're reading fluff text into the intent of the spell that isn't there - there's nothing (other than fluff text) that implies some alien intelligence is suddenly infused and imparted with control of the undead body.

Mechanically, it gives the Fiendish template, no more, no less. The undead is no more under the possession of a fiend than a Fiendish Dire Rat is.

The text about "independent of you" is there only to point out that the spell gives the caster no special control over the undead it is cast on. Similar language exists in several spells that impart new (permanent) abilities to the spell recipient.

..and the bigger concern for an undead PC receiving the spell is suddenly finding yourself with (an extra) +2 LA.

[The Viscount]

The reason I had assumed the fiend takes control is that it states that the undead can take it's own actions, even though mindless beings like skeletons and zombies, which would be under your control, are acceptable targets.

The sudden +2LA from fiendish is a problem, but due to the heavy restrictions on targets for the spell, you're already facing an enormous ECL adjuster beforehand. If you're actually using it it's likely you don't care or have some way around it.

[LTwerewolf]

Abilities: Same as the base creature, but Intelligence is at
least 3.

Part of the fiendish template gives them the ability to think.

[Thurbane]

For mindless creatures that suddenly get an Int score, there is some room to argue that the sentience is that of a fiend (albeit a very dim one).

But then it is also true of a Fiendish Grey Ooze.

[The Viscount]

A fiendish ooze is supposed to be an ooze from the lower plains, or ancestrally part fiend. Vile Death only uses the binding of a fiend because it's applying the usually inherited template to an otherwise invalid target.