When a Synology NAS creates a VPN connection it overides the default gateway, except for devices listed as contained within its subnet. According to the a post on the synology forum even if the subnet is changed to 255.255.0.0 it will not see devices on another VLAN. It feels like a subnet mask of 255.255.255.0 is hardcoded.

For Allowing my Synology NAS to create a VPN connection and see device on my VLANS I had to add a static route defining a gatweway for the VLAN IP/subnet.

Network Destination: 192.168.107.0  # This is the VLAN 107
Netmask            : 255.255.255.0  # Standard netmask for a VLAN
Gateway            : 192.168.100.1  # Gateway for the Synology NASs VLAN
Interface          : physical interface that the Synology NAS uses to connect to the VLAN