Stefan, that have little to do with Raspberry as it's something you have to do at the router/network level.
Linux - so Raspberry - can use the iptables MARK rule to mark packets to be used by QoS enabled routers, but you need a good router capable of throttling bandwidth to achieve anything.
Most cheap/home routers can also tag packets for use with QoS but they just tag the packets and do nothing with them so it's useless unless you have a good router.
The Cisco router I have at work have throttling capabilities. The Linksys wifi access point we have, does have QoS tagging, so we can tag on the linksys and assign priorities on the Cisco.
To build an VLAN - that's not really needed to assign or manage bandwidth throttling - you need an VLAN capable device, and for the Raspberry it will be transparent. It will take its IP from DHCP from that device, and will be routed through that VLAN.
Some routers - as the wifi router WRT54G - can be hacked to put on them firmware capable of bandwidth throttling on cheap hardware - dd-wrt or tomato firmware.
So I don't think VLAN is what you are looking for. Look for a good router with throtling capabilities - you use to set preference by source ip and destination ip so set a DHCP rule so the router always assign that IP to your Pi's MAC - or buy a WRT54G and install dd-wrt on it to be able to shape your bandwidth.
Remember it must be the central device on your network that routes the traffic around. If you get a modem/router with sick shaping capabilities and plug it to a switch where all your devices are plugged, you get nothing out of it, because the switch will redirect the traffic among your devices and the router will only see those packets going out of your network.
There exists switches with throttling and tagging capabilities - like the Cisco Catalyst series - that are quite expensive for home use.
Your best bet is to look for a cheapo router with switch that supports custom firmware, or to look for a switch / routers with throttling capabilities in your price range.
For the NFS server and the Rapberry Pi all this - VLAN or Qos or traffic shaping - will be transparent, and no changes are needed on them.