Public key authentication error -- Unknown file format

[dverbern]

I admit to being pretty stressed and confused at the moment, so apologies.

I'm using VSFTP (VShell) over the command-line. I'm providing options like "-i" and specifying a path in double-quotes to the private key file, I'm specifying "-v" for verbose logging, followed by the remote username the 3rd party has set up for us, "@" the remote host server, then ":" and port 22.

This results in some verbose logging, which ends with:

Public key authentication error.
Could not load the public key from the private key file:
C:\Program Files\VanDyke Software\VShell\PublicKey\{SomeThirdParty}\{SomePrivateKeyName}

Unknown file format.

I've looked this up online and found notes from Brenda and others suggesting that VSFTP employs matching of file names so that the private key could be {Filename} without an extension, so public file should exist in same folder and be {Filename}.pub.

I've followed that guideline, but still fails to improve the situation. Do I need to massage the original private key into some format that VSFTP wants?

The original private key provided to me from vendor has extension ".ppk" and starts with the following lines:

PuTTY-User-Key-File-2: ssh-rsa
Encryption: aes256-cbc
Comment: rsa-key-20190618
Public-Lines: 6

The file goes onto include a "Private-Lines" and "Private-MAC" section.
Most of our other public keys from 3rd parties are much more compact and just start with generic text like "---- BEGIN SSH2 PUBLIC KEY ----
Comment: "rsa-key-20180301""

I'm sorry I am clearly not understanding something. I've got access to the PuTTY Key Generator utility and I've played around with loading an existing private key and generating a new pair, or just saving the public key text to a file, but ultimately I'm not sure what I'm doing or how to progress this issue.

Any help appreciated.