So your phones and other WIFI devices are working but the tablets wont connect?
What happens when you go into WIFI settings on the tablet, forget the connection, turn the WIFI off, then back on, and reconnect tot he WIFI?
Ok, when you go into the WIFI settings when its connected does it show an IP address?
Ok, this does sound likely the router is rejecting the devices.
Appreciate you have rebooted the router, have you tried resetting the router?
How many devices do you have connected to the Wifi (less than 5, less than 10, more than 20)?
This rules out the tablets and points towards a WIFI issue, I would be inclined to ask Vodafone to send you a replacement router which should be free of charge.