Vendor SSH Public key on PI Server

Hello All

Following is my scenario:

PI -- PI SFTP Adapter -- PULL/PUSH -- Vendor SFTP Server

Now we have provided SSH compatible public key (perform the conversion of Public PI X.509 certificate) to the vendor and asked him to copy it under <userID>/.ssh authorized_keys file.

Now vendor also provided their public key to us so i have following questions:

• Do we really need to install vendor's ssh public key in our PI server, as handshake is happening between our PUBLIC/PRIVATE key.
• 
  If we require to install vendor's ssh public key into PI server then with what method we can convert SSH public key into public SSL key (or compatible which we can install inour keystore).

Thanks

Lalit Chaudhary