Most probably, the problem is the base64 format. It is in the form of ASCII characters. You need to convert the Base64 data to binary. And then load the binary data in Workbook class.
' Decode base64
Dim binaryBytes As Byte() = Convert.FromBase64CharArray(base64Data, 0, base64Data.Length)
' Load in MemoryStream
Dim binaryStream As New MemoryStream(binaryBytes)
' Pass memory stream to Workbook
Dim workbook As New Aspose.Cells.Workbook(my_memory_stream)
workbook.Save(dataDir + "workbook-out.xlsx")
Also note that, if your decoding is correct, you can directly save the binary byte or stream to disk and verify that the Excel is saved correctly.
PS. I am a Developer Evangelist at Aspose.