(Forked only to rename file for getting syntax highlighting — I have no C# skills to otherwise improve the code 🙈 :-)

VortoPropertyIndexer.cs

using System.Collections.Generic;
using Moriyama.Library.Architecture;
using Newtonsoft.Json;

namespace Application.Search
{
    public class VortoPropertyIndexer
    {
        // Somewhere in startup
        //ExamineManager.Instance.IndexProviderCollection["ExternalIndexer"].GatheringNodeData += MoriyamaApplicationEventHandlerGatheringNodeData;
        //void MoriyamaApplicationEventHandlerGatheringNodeData(object sender, IndexingNodeDataEventArgs e)
        //{
        //    VortoPropertyIndexer.Instance.AddVortoFields(e.Fields);
        //}   

        public void AddVortoFields(IDictionary<string, string> fields)
        {
            var vortoFields = new Dictionary<string, string>();
            foreach (var field in fields)
            {
                if (!field.Value.StartsWith("{"))
                    continue;

                var jsonString = field.Value;

                var vortoField = DeserialisePropertyToVortoField(jsonString);

                if (vortoField == null || vortoField.Values == null)
                    continue;

                foreach (var vortoValue in vortoField.Values)
                {
                    var newFieldName = field.Key + "_" + vortoValue.Key;
                    vortoFields.Add(newFieldName, vortoValue.Value);
                }
            }

            foreach (var field in vortoFields)
            {
                fields.Add(field.Key, field.Value);
            }
        }

        private VortoField DeserialisePropertyToVortoField(string json)
        {
            VortoField v = null;
            try
            {
                v = JsonConvert.DeserializeObject<VortoField>(json);
            }
            catch { }

            return v;
        }

        private class VortoField
        {
            public IDictionary<string, string> Values { get; set; }
        }
    }
}