You can indeed submit letters of recommendation that are on file with PrivateFolio to *all* application sites, including applications hosted by Interfolio and Vitae. You can also currently transfer the letters that you have on file with us to Interfolio and Vitae by entering the letter's unique email address as the address of your letter writer.
However, there are two concerns that we should draw your attention to:
1) At the moment, Interfolio will not transfer letters to PrivateFolio or Vitae using that same method in reverse (i.e. if you attempt to request a letter of recommendation via PrivateFolio by entering the unique email that Interfolio provides for a letter as your writer's address, Interfolio will ignore your request).
2) We share Interfolio's concern that Vitae is not a secure system (in Vitae, candidates can simply email confidential letters to themselves using a second personal email address). We will always support uploading letters to applications hosted by Vitae. However, we are having an active debate about whether to continue support the transfer of letters to Vitae's dossier service because of the security concern. In the meantime, applicants should be aware that letter writers can view where their letters have been sent, so letter writers will be aware if their letters have been transferred to Vitae.