Arrange rack in lower third of oven. Preheat oven to 350°F. Spread pecans on a baking sheet and roast until they are lightly toasted, about 8 minutes. Let cool and coarsely chop; set aside. Cover same baking sheet with parchment paper; set aside.
To make vanilla syrup, combine 6 Tbsp. sugar and 1/4 cup water in a small saucepan over high heat and bring to a boil; remove from heat, stir in 1/4 tsp. vanilla. Let syrup cool; set aside.
With an electric mixer, beat together Neufchâtel and remaining 3 Tbsp. sugar on high speed until mixture is fluffy. Add flour, egg and remaining 1/2 tsp. vanilla, and mix on medium speed until blended well. Set aside.
Position croissant bottoms on baking sheet and brush each with reserved vanilla syrup. Spread 1/2 Tbsp. jam on each bottom. Spread Neufchâtel mixture over jam, sprinkle with chopped pecans and replace croissant tops.
Bake until croissants are slightly darkened and filling is puffed, 15 to 20 minutes. Let cool slightly and dust with confectioners' sugar, if desired.