The spell-like ability of a blue dragon could do it
Any blue dragon possesses the spell-like ability create/destroy water that, in part, says that
It works like the create water spell, except that the dragon can decide to destroy water instead of creating it, which automatically spoils unattended liquids containing water. Magic items (such as potions) and items in a creature’s possession must succeed on a Will save (DC equal to that of the dragon’s frightful presence) or be ruined. This ability is the equivalent of a 1st-level spell. (Monster Manual 72–3)
(Links added.) As this answer suggests, the 0-level spell prestidigitation [univ] (Player's Handbook 264) is probably the easiest solution if the DM allows it, and, as this answer suggests, the dust of dryness (Dungeon Master's Guide 255) (850 gp; 0 lbs.) is likely the most readily accessible irreproachable method, but it seems that the blue dragon's spell-like ability create/destroy water when used to destroy water could also purge any holy water from a vampire lord's ashes.
However, convincing the blue dragon to use its create/destroy water ability on the vampire lord's ashes is another story… probably a very expensive story, too.
Further, it may be possible, as per answers to this question that if there's a blue dragon that can be dealt with amicably, in conjunction with a craftsman that possesses the feat Craft Wand (PH 93), the blue dragon need only take one day out of its undoubtedly extremely busy schedule and need only trigger one time the spell-like ability create/destroy water to enable that craftsman to create a wand of create/destroy water (1st-level spell at caster level 1) (15 gp/charge; 0 lbs.).
While the only way to activate that wand is by making a Use Magic Device skill check to activate a wand (DC 20)—because not even blue dragons themselves have a spell named create/destroy water on their spell lists—, that's probably the least of an Oh,-noes!-My-vampire-lord's-dead-again servant's problems.