Vestals should actually be pretty good vs. Caelum, by dint of generally fairly average attack, I'd think. At best 12 (and that on cap-only critters with shorter weapons) vs base 16 (significantly more if you go water bless, which is damn sexy on vestals) + shield is... not the best odds. Not the worst, but still. Add to that that the vestals are undead and that makes darkness a go-to for turning the fight from advantageous for sceleria to a complete stomp, well...
Honestly, MA Caelum's best bet vs. vestals may be the blizzard warriors. The high precision may actually do better than the melee fighters. Try to bury them in light infantry, perhaps...
I wouldn't even remotely want to try to take caelum into a straight up fight with sceleria, though, unless it was very early. Lightning evo could do a lot vs. undead hoards, but storm+darkness is kinda' painless for sceleria and kicks caelum right in the teeth. Though getting storm for 'em without pretender intervention would be a little rough. Not exactly impossible -- they've got the potential for A2 and astral rings -- but relatively improbable. Still, it'd probably be a pretty nasty match up for caelum, imo, especially if they didn't manage to get an early handle on sceleria's reanimator production. I'd probably be considering investment into seeking arrow spam...