i'm not sure if i understood this correctly ,
client1 , client2 , server1 , server2 are all connected to the same vpn server as vpn clients , and you want that client1 and client2 can't reach server1 or server2 trough the vpn ?
it's not that hard , but you'll have to implement some type of firewall on your vpn server or possibly you could do that even on server1 & server2 if you don't do NAT on your vpn.
your VPN server should be configured for ease of use with something like
topology subnet
server 10.10.10.0 255.255.255.0
you can use a higher netmask if you want
then dived this network in two separate subnets
10.10.10.0 255.255.255.0 (10.10.10.0/24) => 10.10.10.0-10.10.10.127 255.255.255.128 (10.10.10.0/25)
=> 10.10.10.128-10.10.10.255 255.255.255.128 (10.10.10.128/25)
then set up --client-config-dir (see the manual on how to do that) and assign the clients IP addresses from one range and the servers from another (excluding 10.10.10.0,10.10.10.127,10.10.10.128,10.10.10.255) , then set up firewall rules between the two ranges on your vpn server
you can create smaller subnets , or even do individual IP addresses firewalling rules , take this as just a summary of how you could do this and not sure if i'm 100% accurate .
