You need to define an equivalent structure of the JSON structure in c#.
For example, your structure could look like this:

 

// Root myDeserializedClass = JsonConvert.DeserializeObject<List<Root>>(myJsonResponse);
    public class Root
    {
        public string VendorCode { get; set; }
        public string UTRNo { get; set; }
        public bool IsValid { get; set; }
        public int VendorTypeID { get; set; }
    }

Then you’ll need a library like Newtonsoft JSON to covert the data into a list. The code will look something like this.
 

Root myDeserializedClass = JsonConvert.DeserializeObject<List<Root>>(myJsonResponse);

@bhagyat25, Same as the suggestion by @abhijit.

As I just built an example, posting below

    public class SOOrderEntry_Extension : PXGraphExtension<PX.Objects.SO.SOOrderEntry>
    {
        #region Event Handlers
        public PXAction<SOOrder> action;
         PXUIField(DisplayName = "Actions")]
         PXButton(SpecialType = PXSpecialButtonType.ActionsFolder)]
        protected IEnumerable Action(PXAdapter adapter)
        {
            string jsonDept = @""{""VendorCode"":""V000000001"",""UTRNo"":""T045353"",""IsValid"":true,""VendorTypeID"":1}]";
            var serializer = new JavaScriptSerializer();
            List<VendorData> deptObj = serializer.Deserialize<List<VendorData>>(jsonDept);
            return adapter.Get();
        }

        public class VendorData
        {
            public string VendorCode { get; set; }
            public string UTRNo { get; set; }
            public bool IsValid { get; set; }
            public int VendorTypeID { get; set; }
        }
        #endregion
    }

Added NewtonsoftJson from Nuget and when running following error comes.

Hi @bhagyat25 ,

Can you add the Newtonsoft reference from the Acumatica instance folder/bin folder? 

Added NewtonsoftJson from Nuget and when running following error comes.

 

I suspect there is an existing Newtonsoft.Json being referenced. So you’re getting this message. Try to find that dll and reference it, instead of adding it from Nuget Packages.