John Gill Commentary Psalms 56:12

"Thy vows are upon me, O God: I will render thank-offerings unto thee." — Psalms 56:12 (ASV)

Your vows [are] upon me, O God
Which he had made to him in the time of his distress and trouble, and which he looked upon himself under obligation to perform; they were debts upon him he ought to pay off; they were with him; they were fresh in his mind and memory; he had not forgot them, which is often the case when trouble is over; and he found his heart inclined to make them good;

I will render praises to you ;
which explains what he meant by his vows; namely, sacrifices of praise and thanksgiving to the Lord: when he was in distress, he had vowed and promised, that, if the Lord would deliver him, he would praise his name, and give him all the glory; and now he resolves to fulfil what he had promised.