Actually, if you're vit/dex you don't lose half as much pd as people claim.
Nevertheless, you're better off going evolve mode as of lvl 65 and onward, since you otherwise miss out on a lot of stuff like suction, dragon's nail etc.

Other than that, I thoroughly enjoyed sepa mode until that level and had no issues with my defence at all. Being 3vit1dex I still outtanked and outdamaged most of my peers.. doing minor like that was incredibly fun.

If it wasn't for suction and splitter, my DS would still be in sepa mode.