Users are near us-east1 and europe-west1 and their workloads must communicate with minimal cost and optimal network efficiency. Which topology do you design?

Creating one VPC with two regional subnets (one in us-east1 and one in europe-west1) allows workloads to communicate directly using private RFC1918 IP addresses. This is the most cost-effective and efficient solution because internal traffic within a single VPC, even across regions, uses Google's high-speed private network and is not subject to egress charges for cross-region private IP communication. Using two separate VPCs with VPN gateways introduces additional cost for VPN tunnels and potential latency. Using external IP addresses on instances for cross-region connectivity incurs high egress costs and is less secure. A global load balancer is primarily for distributing external traffic to backend services, not for direct private communication between internal workloads in different regions of the same VPC.