Hi there.
Depends on the abilities of your Draytek.
Let's say your router is 192.168.1.1. So you would define some VLANS on your switch:
vlan 10
name "Office 1"
untagged 2-5
ip helper-address 192.168.1.1
ip address 192.168.2.1 255.255.255.0
exit
vlan 20
name "Office 2"
untagged 6-9
ip helper-address 192.168.1.1
ip address 192.168.3.1 255.255.255.0
exit
...
Then you have to turn routing on:
ip routing
And now you can define access-lists which permit or deny connections between the pcs.
Your router has to be able to give your pcs ip addresses within the right range. For example: if the pc is one of Office 2 it has to use a ip address 192.168.3.4 (or something like that). The gateway address for this pc has to be 192.168.3.1
If the draytek is not able to do this allocation by vlan (ip address of the dhcp relay) you have to use fixed ip addresses for your clients or use a seperate dhcp server (under linux?).