question

It is first important to realize an API call can be setup nested (with lambda or " => ") or as separate methods like this:

public void GetMyInventory()
{
	// Build the request, Then pass it to API call

	var request = new GetUserInventoryRequest(); //gets all results
						     
	PlayFabClientAPI.GetUserInventory(request, GetMyInventorySuccess, 
        	GetMyInventoryError); //make API call
}

private void GetMyInventorySuccess(GetUserInventoryResult result)
{
	Debug.Log("GetUserInventorySuccess: " + result);
	Debug.Log("Do Something With Inventory Here!");
}

private void GetMyInventoryError(PlayFabError error)
{
	Debug.Log("GetUserInventoryError: " + error);
}

VirtualCurrency can be accessed as a batch with "PlayFabClientAPI.GetPlayerCombinedInfo" as you suggested or you can call just "PlayFabClientAPI.GetUserInventory".

You can have access to both if needed. So, my method is nested with lambdas (=>) and "GetUserInventory" looks like this:

public readonly Dictionary<string, int> virtualCurrency = 
	new Dictionary<string, int>();

public readonly List<ItemInstance> playerInventory = new List<ItemInstance>();

// This is a sorted, collated structure built from playerInventory.
// By default, this will only grab items that are in the primary catalog
public readonly Dictionary<string, InventoryCategory> inventoryByCategory = 
	new Dictionary<string, InventoryCategory>();

public void GetUserInventory(Action callback = null)
{        
    PlayFabClientAPI.GetUserInventory(new GetUserInventoryRequest(), 
	(GetUserInventoryResult result) =>
    {
        virtualCurrency.Clear();     // Local Dictionaries for VirtualCurrency & 
        playerInventory.Clear();     // InventoryItems
        inventoryByCategory.Clear();

        foreach (var eachPair in result.VirtualCurrency)     //VIRTUAL CURRENCY//
        {
            virtualCurrency.Add(eachPair.Key, eachPair.Value);
            Debug.Log("Virtual Currency: " + eachPair.Key);

            if (eachPair.Key == "GC")
            {
                Debug.Log("GC currency found");
                MessagingData.UpdateCurrencyValue(eachPair.Value);
            }
            else
            {
                Debug.Log("GC currency was not found!");
            }
        }

        foreach (var eachItem in result.Inventory)           //PLAYER INVENTORY//
        { 
            playerInventory.Add(eachItem);
            Debug.Log("Inventory Item: " + eachItem.ItemId);
        }

        if (GameDataManager.Instance.catalogItems.Count > 0)
        {
            foreach (var item in playerInventory)     //PLAYER INV. BY CATEGORY//
            {
                if (inventoryByCategory.ContainsKey(item.ItemId))
                    continue;

                var catalogItem = GameDataManager.Instance.GetCatalogItemById(item.ItemId);
                if (catalogItem == null)
                    continue;

                var items = playerInventory.FindAll(x => 
			{ return x.ItemId.Equals(item.ItemId); });
                
                inventoryByCategory.Add(item.ItemId, 
			new InventoryCategory(item.ItemId, catalogItem, items);
            }
        }

        if (callback != null)
            callback();
        
    }, (PlayFabError error) =>
    {
        Debug.Log("Get User Inventory Error: " + error);
    });        
}