I'm not 100% convinced Unholy Vampire exists.
The only definitive references I can find to this variant are from a pair of posts, both by the same user, in an old character optimization contest (the first one is the one linked in the question). Since they don't provide a source, and I can't find any mention by anyone else, it may just be something that player or their table homebrewed (though it would be weird to use something like that in a contest).
If it exists, it's probably in Dragon Magazine somewhere.
Unholy Vampire definitely doesn't appear in any of the places I know of with variant vampires, such as Libris Mortis and Dragon Magazine #348. However, the Gestalt Build Challenge (the optimization contest in which the variant was mentioned) is notable for explicitly allowing Dragon Magazine content. If you find something used in a contest that allows Dragon material, and it turns out to be really hard to Google, there's a good chance it's Dragon material.
Unfortunately, after a medium-thorough scan of the tables of contents and likely-looking features from the entire 3.0/3.5 run of Dragon Magazine, I haven't found it. That doesn't mean it's not there; I didn't fully read every single issue and it's quite possible I missed it.
Other possibilities: Templates with names other than "Unholy Vampire."
Although Belial_the_Leveler lists the template/creature as "Unholy Vampire" in both places he mentions it, it's possible that he's referring to something else, such as an "Unholy Creature" template that just happens to be modifying a Vampire in this case (this would be consistent with their use of "Evolved Vampire" in the table, which is referring to the "Evolved Undead" template from Libris Mortis, p. 99).
Unfortunately, I haven't been able to find any such template. The closest I have been able to find is a Pathfinder template with the same name from the Advanced Bestiary, but it doesn't fit (doesn't provide Unholy Toughness, not LA +1).
Again, it may be somewhere in the Dragon Magazine back catalogue.
If you're specifically looking for the effect of the Unholy Toughness ability, you might have some options.
Unholy Toughness is an ability found mostly in Libris Mortis and Monster Manual 3, which allows undead creatures to substitute their charisma bonus for their constitution bonus (which, as undead, they don't have) when calculating their HP.
Unfortunately, it mostly appears on very specific undead creatures, which aren't the kind of thing you can tack onto an existing character.
If you're looking for extra HP for your undead character, you might consider:
- The Walker in the Waste PrC (Sandstorm, p. 89), which grants as its capstone the Dry Lich template, which has Unholy Toughness as a feature.
- Somehow being turned into an undead in an area affected by Desecrate, which would grant you +1 HP/HD. This is obviously not an option for a character who is already undead.
- Faerie Mysteries Initiate feat (Dragon #319), which allows you use Intelligence in place of Constitution when calculating HP. It may be something of a challenge to justify the Fae connection for this feat fluff-wise on an undead character.