How to programmatically handle files (CSV import)
Drupal's naming convention for the automagic association of validation and submit handlers for a given form is:


name_of_form() (builder)
name_of_form_validate() (validation handler)
name_of_form_submit() (submit handler)


In your case, if you wanted to name your form vts_departs_conf_form(), you'd want to either rename your validation handler to vts_departs_conf_form_validate() and your submit handler to vts_departs_conf_form_submit(), or use the #validate and #submit form attributes to associate functions that don't follow the naming conventions.

In your example, changing the builder name to vts_departs_conf() made the form work as expected.