CO2 concentrations are not calculated from proxies; they are directly measured from bubbles trapped in the ice, and therefore are probably quite accurate. The temperature is determined from stable oxygen isotope ratios and is more uncertain.
But here is an interesting statement ---"Because air bubbles do not close at the surface of the ice sheet but only near the firn-ice transition (that is, at ~90 m below the surface at Vostok), the air extracted from the ice is younger than the surrounding ice (Barnola et al. 1991). Using semiempirical models of densification applied to past Vostok climate conditions, Barnola et al. (1991) reported that the age difference between air and ice may be ~6000 years during the coldest periods instead of ~4000 years, as previously assumed."
Link --- first item in Google search on "Vostok ice"
I need to ponder this statement. Have any thoughts on this?
This is further down in the link ---"According to Barnola et al. (1991) and Petit et al. (1999) these measurements indicate that, at the beginning of the deglaciations, the CO2 increase either was in phase or lagged by less than ~1000 years with respect to the Antarctic temperature, whereas it clearly lagged behind the temperature at the onset of the glaciations. "