Why does Vanka ask his grandpa to come to Moscow?

In his letter, Vanka explains to his grandfather that he will be dutiful and hardworking if he will take away from Alyakhin. Vanka promises that he will grind Konstantin Makarich's snuff, and that he is willing to clean boots or work as a shepherd to prove his worth. Life at Alyakhin's is torture to Vanka, who (as he explains) has only stayed on because he has no boots and is fearful of the cold. Vanka then pledges that, if he is brought back to the village, he will grow up to protect and pray for Konstantin Makarich. He wants his grandfather to take him away.