Well stated Gertjan - but would like to clarify one small part..
Your not limited to the 1 (one) lan side port with vlans.. If you have multiple ports you can use them for multiple lan side networks.. As long as you have a switch you can connect your multiple ports from the router as different networks tagged or untagged.
Router ports or interfaces are very valuable on a router - trying to bridge them to put in the same layer 2 network is waste of time, effort to end up with a more complex setup and less performance.. You might only do such a thing when you want to connect interface types… Say you had a fiber connection on your router and you wanted this fiber connection to be in the same L2 network and as normal copper ethernet network.
Or if you wanted to split the same L2 network but be able to firewall beside your split ends..
There are for sure uses of bridging interfaces on your router - but unless your doing something that is a bit more complex and requires such a setup... It going to be a much better idea to get just a vlan capable switch when you need more "ports" leverage your routers interfaces as switch ports via bridging them not a good idea almost ever.
If you need a few ports in the same network and you want them to be on your router box - get say the sg-3100, it has a 4 switch ports that you can use a switch in the same network or break them out as individual vlans, etc.
But you can for sure get a 8 port get switch that is smart and easy to configure for like $30... If you want multiple interfaces on the same network - your way better off getting a switch then trying to bridge your very useful router interfaces.. It would be better to let those interfaces just sit unused for future use then try and leverage them as switch ports. If your ocd and don't like these router ports left open - then use them in a lagg to your shiny new switch ;) Be it your using 1 network or multiple vlans.