Put the dried pears in a small heatproof bowl. Cover with 1/2 cup boiling water and let stand until plump, 5 to 10 minutes.
Toast the mustard seeds in a dry medium saucepan over medium heat, tossing occasionally, until darkened (some may start to pop), about 5 minutes. Add all but 1 cup cranberries to the pan along with the sugar, vermouth, a pinch of salt and the pears with their soaking liquid. Bring to a simmer and cook, stirring occasionally, until the cranberries burst and the sauce thickens, 15 to 20 minutes.
Stir in the reserved 1 cup cranberries and 1/2 cup water; simmer until the cranberries soften, 3 to 4 minutes. Transfer to a bowl and refrigerate until cooled, about 2 hours.
Stir together the mustard powder and 1 teaspoon cold water to make a paste. Stir into the cranberry sauce. Refrigerate until cold, at least 2 more hours and up to 1 week.