The three flavors of the updated mod is based on Squinge's original mod, GrandVampireOften.
25% lets the Grand Vampire appear slightly more often.
50% lets the Grand Vampire appear half the time.
75% lets the Grand Vampire appear at least 75 percent of the time. (This was the setting for Squinge's original mod.)
The default setting for the Grand Vampire without the mod is 15%, meaning that the Grand Vampire rarely shows up. You would be lucky to see one downtown.