Chop them in small cubes and toss them in a bowl with the lemon zest and lemon juice. Set aside.
In a large heavy saucepan combine the apple juice concentrate with the vanilla bean and the water.
Boil it over moderate heat for 10 minutes.
Remove the vanilla bean and add the pears and their liquid.
Bring the mixture to a boil over moderate heat and cook, stirring frequently, about 45 minutes, or until the mixture thickens enough to hold its shape on a spoon and has reached nearly 220 degrees F. on a candy thermometer.
Spoon the jam into prepared jars leaving a 1/4" headspace.
Process in a boiling water bath for 10 minutes. Adjust time according to your altitude.