I'll probably get downvoted for this but I dont think these trials can conclusively say that your chance of getting dust from a Vampire differs from type to type. The samples are different sizes and and the last two (Vampire Bride and Vampire Viscount) could well fall within the margin of error for being the same as the first (Vampire)
I think a larger sample size for both Vampire Bride and Vampire Viscount is needed before we can say the chance of dusting is different between types of Vampires