This is one of the flaws of statting out beings that probably shouldn't have been statted in the first place. Planescape, back in 2nd Edition, was really good about this, indicating that the Powers, and even the Lords of the Nine or the demon princes (the Celestial Hebdomad didn't come into existence until the Book of Exalted Deeds, in an attempt to give an Upper Planar equivalent to the Lords of the Nine; the same goes with the other celestial paragons, like Morwel, logically concluding that if there were demon princes in the Abyss, there would be Upper Planar equivalents) weren't statted out, with the exception of Graz'zt and Pazuzu.
This being said, one major difference exists between the Book of Vile Darkness and the Book of Exalted Deeds: The former assumes the combative prowess of the entities contained within, violence being an assumed function of Evil, while the latter assumes that while they're capable of combat, martial pursuits aren't the primary focus of the Upper Planar rulers. Chances are that if, for some reason, Zaphkiel and Asmodeus met in combat, they'd be pretty evenly-matched, but that isn't really the point. Zaphkiel isn't likely to leave Chronias to get in a fight with Asmodeus in the first place, and most of the time, players are playing non-Evil characters, which means they have little reason to attempt to invade Chronias (or any of the other six layers of Celestia), which means they have little reason to bring the fight to Zaphkiel in the first place.
Don't look at the celestial paragons as things to be fought, unless you're playing an Epic evil campaign (and of course, those are out there...). Most of the time, the PCs ought not to even encounter one of the celestial paragons. They're too busy ruling Upper Planes, and the races native to them. What, do you want your Charisma 38 Paladin to pull out rules from the "Book of Erotic Fantasy" and attempt to make a play for Morwel? Then you might have a fight on your hands, as she has two consorts who are likely quite happy to have her as their partner, but even then... Chances are, you'd just be ejected from the plane.
In other words, there isn't much reason for the celestial paragons to be huge combat tanks. In about 80% of the campaigns out there, they aren't going to be fought in the first place.
But the Lords of the Nine or the demon princes? That's another barrel of fish entirely...