Exoskeleton is no different to haki. Your base defence is just as important as your extra defence. Only difference is different people’s haki varys in strength. Whilst Exoskeleton gives the same boost.
Say Sanji’s base durability was 100, and the Exoskeleton gave him a 100 boost in durability.
Then the Vin bros base durability would be like 10, whilst the Exoskeleton would give them 100.
Meaning Sanji would be 200, whilst they would be like 110.
These are estimates obviously, but just to give you an idea of how it would work.