Vault uses Microsoft Save To PDF engine to create renditions. If a Powerpoint file is not rendering in Vault, first try saving it in separate sections to find the bad slide(s). 

1. Click File --> Save As --> Browse.
2. Choose to Save a set of the slides.  For example, if the document has 10 slides in it, save 1-5 first, then save 6-10.
3. Change the file type to PDF.
4. Click Save. If the save fails, that section of slides has the bad slide in it.
5. Then, save a smaller group of the failed group. Continuing until the one bad slide is left.

 Once the bad slide is found, export it to a different file type. To Do this;

1. Click File --> Export --> Change File Type.
2. Double-click the type of file to Save as, and choose a new file name.
3. Click Save and when prompted, click Just This One.
4. Delete the bad slide from the presentation.
5. Add the new one that was just saved back into it.
6. Re-upload the presentation to Vault.

Related Documentation:

Vault Rendition Documentation: Working with Renditions