Shared memory between c# and c++

I'm working on a project to create a VR sculpting tool voxel based, this project that involved two main process. On Unity I manage the user interaction and the storage of data structure. For the data manipulation I use c++ libraries. The voxel grid is very big, about 4000x4000 voxels, I need more efficiency, this is the reason why I chose to use the c++ dll. I have read more tutorial e docs about the intoroperability berween c# and c++, but I'm still confused about how to implement the comunication from two process. I have to manipulate two structure, one fixed size array of int (c#: int[] -> c++: int[]) and one variable size vector of pointer to a vector (c#: List<IntPtr> -> c++: std::vector<std::vector<int>*>). Up till now, I've been able to read\write from the int[], I have some problem for the second one, I can't add element to the vector from c++ dll, if I add an element from c# I don't have problems to read and write it from c++. The code of the two processes:

 /************    C++  *************/
 #define EXPORT_API __declspec(dllexport) 
 
 //Function to write into a int[] array
 EXPORT_API bool GetIndices(int indices[], int indicesLenght) {
 
     for(int i = 0; i < indicesLenght; i++){
         indices[i] = 10;
     }
 
     return true;
 }
 
 static void* blocksBufferHandle = NULL;
 static void* blocksBufferHandleOriginal = NULL;
 static int blocksBufferVertexCount;
 static std::vector<int*> g_VertexSource;
 
 EXPORT_API void SetBlocksBuffersFromUnity(void* blocksBufferHandleFromUnity, int blocksCount)
 {
     blocksBufferHandle = blocksBufferHandleFromUnity;
     blocksBufferHandleOriginal = blocksBufferHandleFromUnity;
     blocksBufferVertexCount = blocksCount;
 
     
     g_VertexSource.resize(blocksCount);
     for (int i = 0; i < blocksCount; ++i)
     {
         auto *b = new std::vector<int>(16, 5);
         g_VertexSource[i] = b->data();
     }
 }

 /************    C#   *************/
 using System;
 using System.Runtime.InteropServices;
 using System.Collections.Generic;
 using UnityEngine;
 
 
 public class PluginImport : MonoBehaviour
 {
     
     [DllImport("libUnityPlugin", CallingConvention = CallingConvention.Cdecl)]
     public static extern bool GetIndices([MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1)] IntPtr indices, int indicesLength);
 
     [DllImport("__Internal")]
     private static extern void SetBlocksBuffersFromUnity(IntPtr blocks, int blockSize);
 
   
     public int size; //Grid size
 
     void Start() {
       
         int indicesLenght = 10;
         int[] indices = new int[indicesLenght];
 
         GCHandle indicesHandle = GCHandle.Alloc(indices, GCHandleType.Pinned);
         IntPtr addressIndices = indicesHandle.AddrOfPinnedObject();
 
         GetIndices(addressIndices, indicesLenght);
 
         Debug.Log("Indices value: " + (int)indices[0]); //Output: 10 OK
 
         int blocksLength = 10;
         var blocks = new List<IntPtr>();
 
         GCHandle blocksHandle = GCHandle.Alloc(blocks, GCHandleType.Pinned);
         IntPtr addressBlocks = indicesHandle.AddrOfPinnedObject();
         SetBlocksBuffersFromUnity(addressBlocks, blocksLength);
         
         Debug.Log("Block value: " + blocks[o]); //Error
        
 
         indicesHandle.Free();
         blocksHandle.Free();
 
     }
 }

If I try to read the first element Unity of the vector, returns this error, If I print the length of blocks is 0 :

 ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.

I suppose c# doesn’t notice the addition of elements made by c++. For some parts of the code (Setblocksbuffersfromunity) I took the cue from the Unity documentation. I'm working on this code from about 2 month, I tried several solutions, but any worked for the vector. Any suggestions? thanks