Overview:
A viewable rendition downloaded from Vault does not have a manifested e-signature after going through a workflow that created one.
Root Cause:
The source Document likely has a password or Security is enabled on it.
Solution:
Vault cannot manifest an e-signature on a Document that has security on it. The security has to be removed and the workflow re-run in order to capture an e-signature. The Screenshot below is an example of a secured document.
Related Documentation:
Vault Help Documentation: N/A