I agree with Britton Smith pointing out that the critical value depends on z

I agree with the colleagues. One can use fitting formulas which are available in the literature for a large family of DE models. Check a) Weinberg & Kamionkowski 2003, ApJ, 341, 251 (see their eq.18) b) Basilakos, Spyros; Sanchez, Juan Carlos Bueno; Perivolaropoulos, Leandros, 2009, Physical Review D, 80, 043530 (see their eq.5.16) c) Douglas et al. 2008, ApJS, 174, 277 . The latter papers give the Dvir(z), if you need the critical Delta_c then you have to use: Delta_c(z)=Omega_{m}(z)\Delta_{vir}(z). Of course at large redshifts in which \Omega~1 the two Delta's almost coincide.