The allowed IP ranges on the server indicate what private addresses the clients can use, so you should have a separate one for each client. They can be /32 addresses as each client only needs one address and, I’m assuming, doesn’t route traffic for anything else.
The allowed IP range on each client indicates what private address the server can use, but as the server is also routing traffic for other machines (the other client for example) it should cover those too.
Apologies that this isn’t better formatted, but I’m away from my machine. For example, on your setup you might use:
On home server: AllowedIPs 192.168.178.0/24 Address 192.168.178.2
On phone: AllowedIPs 192.168.178.0/24 Address 192.168.178.3
On VPS: Address 192.168.178.1 Home server peer: AllowedIPs 192.168.178.2/32