Hello Chandra,
What type of site-to-site VPN are you talking about? Is it a dedicated appliance, or is it associated with a security appliance like a firewall with VPN capability?
With this, we have information on how to capture logs from your site-to-site VPN. You can also review the log collection capabilities below for more information on this.
Ref:
With this information, I will be able to provide guidance on the integration.