VPN connections are point-to-point connections in encrypted tunnels. On a point-to-point connection, an "all (binary) ones" (i.e. /32, 255.255.255.255) subnet mask is correct.
A /24 subnet mask (255.255.255.0) would only be correct if you had up to 254 devices on the same shared-medium data-link-layer (layer 2) network, such as an Ethernet LAN.
If your VPN clients can't talk to each other, it's because your VPN server isn't being a good router and routing the traffic correctly between the VPN links. Look at your route tables and firewall rules and your VPN server software configuration and see if you can determine why packets aren't being routed between VPN links.