Slower and less numerous than Goblin and Goblin+, the other Thunder Blitz unit of note, as well as being a larger target, nevertheless the Blood Sucker is frequently the better choice for handling higher defence stats or freeze users. They'll also typically come out on top against goblins, which the AI will frequently send out against them.
Quest 6-6: Unlike Goblins, Blood Suckers are tall enough to hit the boss, and can handle the goblins the boss counters with.
Quest 8-6: Being Freeze immune, the boss can do little against them, while her goblins again tend to lose against the leeching vampires.
Like all Blitz units, Blood Sucker Vampires are easily taken out by ranged units they're not resistant to, and any melee they don't have an advantage against is likely to come out on top.