http://www.sparkviewengine.com/documentation/viewlocations#Understandingthesubfolderparameter

SubViewFolderTester.cs

    public class SubViewFolderTester
    {
        [Test]
        public void create_subviewfolder()
        {
            var settings = new SparkSettings();
            var folderSettings = new Dictionary<string, string>
                                     {
                                         {"virtualBaseDir", "~/Masters"},
                                         {"subfolder", "layouts"}
                                     };

            settings.AddViewFolder(ViewFolderType.VirtualPathProvider, folderSettings);
            var factory = new SparkViewFactory(settings);

            var viewFolder =(CombinedViewFolder) factory.ViewFolder;
            Assert.IsInstanceOf(typeof(FileSystemViewFolder), viewFolder.First); // Pass
            Assert.IsInstanceOf(typeof(SubViewFolder), viewFolder.Second); // Fails, is of VirtualPathProviderViewFolder type

        }

        [Test]
        public void create_subviewfolder_using_hack()
        {
            var settings = new SparkSettings();
            var folderSettings = new Dictionary<string, string>
                                     {
                                         {"virtualBaseDir", "~/Masters"},
                                     };
            settings.AddViewFolder(ViewFolderType.VirtualPathProvider, folderSettings);
            settings.ViewFolders.Last().Subfolder = "layouts";
            var factory = new SparkViewFactory(settings);

            var viewFolder = (CombinedViewFolder)factory.ViewFolder;
            Assert.IsInstanceOf(typeof(FileSystemViewFolder), viewFolder.First); // Pass
            Assert.IsInstanceOf(typeof(SubViewFolder), viewFolder.Second); // Pass
        }
    }

I've run the tests against the spark.web.mvc version and got the same results.