We've had Bergkamp and Henry both of whom were two of the best we'll ever see in Arsenal colours. We replaced them and I don't think we were weaker for it, we were simply different. Cesc subsequently became the new talisman and I anticipate that someone else will pick up the baton once he's left.
When we boil it down, Cesc's greatest contribution to our team is what he adds to our attack but there's more than one way to skin a cat and we can find different but equally potent ways of going about our business. If we're talking players who we've struggled to replace in terms of contribution to our team, I'd plump for Vieira.