You may be able to achieve split DNS (conditional forwarding) with the following configuration (assuming ppp0 is your VPN interface):
resolvectl dns ppp0 corp.ip.add.ress resolvectl domain ppp0 ~corp.domain.name This will use the default DNS for all queries except for those that have a domain ending with corp.domain.name. For those queries, it will use corp.ip.add.ress.