I am sure how you were able to get the dataadapter to fill a datatable. It should be filling a dataset. It should be
DIM ds as New Dataset()
da = New MySql.Data.MySqlClient.MySqlDataAdapter(sql, DBconn)
da.Fill(ds)
DIM rowCount = ds.Table(0).Rows.Count
Try this
DIM rowCount = ds.Tables(0).Select().Count