So as far as I know the rob of Vecna is clearly the best set of robes, then what? archmagi perhaps? Are there any other special robes or are they all just the usual generic robes?

The enhanced edition adds at least a couple of robes. Most notably is the Robe of Goodman Hayes, which confers permanent Chaos Shield for a wild mage. There is also a Robe of Invocation with excellent all-round elemental and acid resistance.

That's so nice to hear, hickory. I usually equip Aerie with Vecna robe, but my second mage is left with low power equipment. Good thing EE fix this problem, even though I haven't found these new robes... What is their location?

The Robe of Goodman Hayes is acquired during Neera's quest for freeing the wild mages. The other cloak you get from the Order of the 8 Staves stronghold.

item upgrade mod adds some, if that's your thing

Next best robe would be Robe of the Good Archmagi (Base AC 5) from the Smithy in Trademeet for 15,200 gold; or for free in Watchers Keep on Level 3. (Or buy the other versions to suit your alignment: Neutral, or Evil from the Underdark merchant in Ust Natha).

The Robe of the Good Archmagi and Vecna (12,600 gold from Deidre in Adventure Mart) are the only robes you need for all of SoA and ToB! And in SoA you can easily delay acquiring these until Chapter 6 - you will be fine.