Hello again,
I've set ports 1-7 PVID to 10. Is it necessary ?
Being more familiar with Cisco switches and never having configured a D-Link switch, I don't know how a D-Link switch reacts if you configure PVID x and an untagged VID y for the same port, where x and y are different. In theory there can only be one untagged VID per port, hence only x=y should be possible, and the switch should refuse any trial to configure x<>y for any port.
On the other hand within "VLAN > 802.1Q VLAN PVID" you can't circumvent to set some PVID per port, hence setting PVID=10 for ports 1-7 is at least consistent with setting ports 1-7 to untagged for VID 10 within "VLAN > 802.1Q VLAN" and obeys to the "x=y" rule discussed above.
Since you configured VIDs 10 and 50 both tagged and no additional untagged VID for port 8 within "VLAN > 802.1Q VLAN", in principle you could configure a PVID x other than 10 or 50 for port 8 (e.g. PVID=1), but this would not make much sense, since VID x is not available on any other switch port. A setting of PVID=10 for port 8 just means that frames entering port 8 untagged would be tagged 10 inside the switch. Of course this case never happens, because your Sophos device plugged to port 8 only sends tagged data (VID 10) or voice (VID 50) frames.
... is it really usefull to deal with Vlans, or running everybody in Vlan 1 would make no difference ?
Vlans are useful if you want/must have several LANs inside which different types of traffic have to flow strictly seperated from each other due to several possible reasons, for example security or different QoS settings (as is the case with you where voice frames have to be preferred to data frames).
Without Vlans you would have to use one physical switch per seperate LAN to isolate traffic from each other. With Vlans (and that's the joke) you can use one physical switch and nonetheless seperate traffic from each other, because frames are tagged with different VIDs inside the switch and no frame travelling inside VLAN x can never get to VLAN y. With Vlans you can imagine your physical switch being partitioned into several logical switches, one logical switch per VLAN, where the logical ports of each logical switch are mapped to the physical ports of your physical switch, and where several logical ports of different logical switches may share the same physical port (externally distinguishable because in this case of "overloaded" ports frames must be sent tagged with the VID of the VLAN they belong to, with the exception of exactly one VLAN, that is allowed to send untagged frames).
If there are no reasons to separate data of different types (same security level, no different QoS demands, same characteristics for any other quality aspect), you don't have to use Vlans, or to be more accurate, you only use one Vlan (which is default VID 1 untagged on every port).
PT