I’ve noticed some distress on the web because, with the release of Synology DSM version 6.0, it is no longer possible to use the vconfig command. This command was used to configure VLAN tagging on your interfaces.
It is however still perfectly possible to create multiple sub-interfaces on a physical interface or bond without using the vconfig command. All you need to do is create additional config-files for each of you sub-interfaces. Each sub-interface represents a VLAN ID. The config-files are found in /etc/sysconfig/network-scripts/.
Note: shell access is required to your Synology. So you should enable SSH for instance.
In the example below, you will see my Synology has a bond using eth0 and eth1. My setup required to have some additional VLAN tagged sub-interfaces on top of my physical bond interface.
As you can see, I have a sub-interface for VLAN 100, 120, 130 and 20. You only need to copy a config-file using the naming format ifcfg-<phy int>.<vlan-id>, and adjust it to your needs. A (copied) config-file looks like this: