John Gill Commentary Deuteronomy 32:47

"For it is no vain thing for you; because it is your life, and through this thing ye shall prolong your days in the land, whither ye go over the Jordan to possess it." — Deuteronomy 32:47 (ASV)

For it [is] not a vain thing for you
That is, the law is not, and the observance of it in them and their children; it was no light and trifling matter, but of great importance and consequence, obedience to it being attended with rewards, and disobedience with punishment:

because it [is] your life :
if obeyed, the means of a comfortable and happy life, in the enjoyment of all good things, of the preservation and continuance of it to a length of time; and long life was always reckoned a great temporal mercy:

and through this thing you shall prolong [your] days in the land
to which you go over Jordan to possess it ;
That is, through their obedience to the law they should live long in the land of Canaan, to which you go over Jordan to possess it. This explains what is meant by its being their life; their comfortable living and continuance in the land depended on their obedience to the law; see (Isaiah 1:19Isaiah 1:20) .