You are setting up a high-availability VPN to your on-premises network, but the VPN connection fails to establish. You have full administrative access to both the Google Cloud networking environment and the on-premises firewalls serving as VPN devices. The Google Cloud console displays "Negotiation failure" and "BGP is down." Upon checking Cloud Logging with the query resource.type="vpn_gateway" and resource.labels.gateway_id="TUNNEL_ID_NUMBER", you observe frequent log entries in Logs Explorer:

logName: …/logs/cloud.googleapis.com%2Fipsec_events  
type: "vpn_gateway"  
textPayload: "received NO_PROPOSAL_CHOSEN notify, no CHILD_SA built"  

How should you troubleshoot the VPN failure and resolve the issue based on these Cloud Logging entries?