Place a rack in the middle third of the oven; preheat to 350 F. In a large bowl, stir together the white and whole wheat flours, Rumford Baking Powder, and salt until well blended. In a deep, medium bowl, stir together the margarine, honey, sugar, milk, and vanilla until very well blended and any sugar lumps dissolve. Stir the honey mixture into the dry ingredients until well blended; if necessary, stir in just enough more milk to form a smooth dough.
Divide the dough in half; shape each half into a plump log. Roll out each log between sheets of baking parchment into a 1/8-inch thick, 11 1/2- by 14-inch rectangle; roll out from the center in all directions so it is evenly thin all over. Peel off and discard the top sheets of parchment. Using a pizza cutter or a large knife, cut each dough rectangle lengthwise and crosswise into four strips so each portion yields 16 whole crackers plus uneven scraps around the edges. Using the tines of a fork, prick 6 evenly-spaced decorative sets of holes in each cracker. Transfer the paper and dough (including the scraps) to large baking sheets.
Bake (middle rack), one sheet at a time, for 13 to 15 minutes or until the dough sheets are set but still not firm in the center. Set the pans aside until the crackers are cool and firm enough to handle. Reduce the oven to 275 F. Place the scraps in the center of the pan and the crackers, slightly separated, around the perimeter. Place the pans, staggered and on 2 racks, in the oven and toast for 15 to 20 minutes, until the crackers are firm and just beginning to tinge with brown. Remove the pans to cooling racks. Let the crackers stand until cooled thoroughly, then pack airtight. Crush the scraps and use when graham cracker crumbs are called for.