If your two networks are physically connected to each other (share the same wires and switches, not separated by a router), you can configure a subnet mask that makes all your hosts part of the same network… eg a Class B subnet mask (255.255.0.0).
If your networks are physically separated, then you can configure a multi-homed device connecting the two networks. If you select a bridging device, then you should configure a subnet mask like in Solution 1. But, if you connect your networks with a router, then you’ll likely want to configure a Class C subnet mask (because your IP addresses are also part of the Class C Private Network range). Depending on how you position your gateways, the actual physical setup and whether you want any/all hosts to have access to the Internet will determine how you configure Default Gateways, gateways and routing tables.
Depending on how you configure your VPN, LAN B still needs to know how to leave your network. You likely need to configure a DG either to LAN A (most likely). That way for any LAN B hosts, any packets destined for an unknown host would be routed to LAN A, and if the destination is still unknown would be routed through LAN A’s DG. Assumes you’re configuring a Class C default subnet mask for both LAN A and LAN B. But would be different if you’ve configured a VLAN.