DR vs Magic Weapons
First, a weapon has to have an enhancement bonus of at least +1 before any other magical enhancements can be placed upon it, so your Axiomatic only blade should not be able to exist at least per RAW, unless the DM deems otherwise.
In 3.0, if I remember correctly, only the enhancement bonus counts. So yes, (in 3.0) a +1 Vorpal Great Sword against a creature with DR 10/ +2 would still be subject to the full DR.
In 3.5 its resistance would be something akin to DR 5/Magic, so as long as it has any enhancement bonus, either permanent or via a spell such a Magic Weapon, it will pass by its DR.