You would include it in your annual income, and you would submit your VA award/benefit letter as evidence of that income. Also, on the I-864, you must either enter the amounts from your last 3 years of tax returns OR you can explain (on the I-864 or a letter) why you were not required to pay taxes for those years (taxable income below filing threshold, for example).
You can get a VA benefits letter from Ebenefits web site.