They've never replaced Vieira and Sol Campbell. That was, 10 (?) years ago now, and the only thing you can really hold against Wenger. He needs a midfield destroyer and a solid center back before they can aspire to anything higher than challenging for 4th anytime soon.
Personally, Aguero is the better player. Citeh are still going through their early Abramovic Chelsea phase of just buying all the popular names in European football rather than what they actually need or who would fit their system. Why would you swap Sanchez for Aguero? Sanchez will want a metric fuckload of cash and is not really any better. Plus, give it 1, maybe 2 seasons then Barca / Real come calling and he'll be in the papers every other week bitching and moaning until he goes, then they'll have to piss more money away on the next next big thing. If it aint broke, don't try and fix it. Unless the fix is Zidane, then even if you have Sherwood, fix it..