The LA is so much higher than the CR adjustment because the Vampire has a LOT of abilities the PC's can abuse for their entire adventuring careers, while "monster" Vampires can only abuse them 1-3 times before the PC's finally defeat them.
Fast Healing is a common culprit. Monsters with Fast Healing may last a couple more rounds. PC's with Fast Healing can pause for a few minutes after most encounters and meet the next one with full HP. Dominating people and assuming gaseous form at will are pretty powerful too. A smart vampire will rarely have an actual need to fight... let the beefiest enemy with a poor Will save do the messy stuff!
PC's can bypass many of a vampire's limitations with a small amount of cleverness. A Bag of Holding for the coffin makes them much more mobile (they can even sleep inside the coffin in the bag if they want, since air isn't a problem).
In short, playing a vampire as a thug does not make full use of the +8 LA. A vampire rogue with a dominated fire giant is much closer to what you ought to be looking at here!