"Why should we assume they evolved this way and were not created? "
Why would an "intelligent designer" make the same design error in only two lines of heredity and not in all of them?
If you assume God as the intelligent designer and that He is omniscient, it is obvious that the Vit. C deficiency is naturally occuring through evolution.
Sceptism must be framed by scholarship.
Again the qualifier: I'm really not knee-jerkingly against the idea of man and chimp having common descent.
But the creationist argument has as much weight as yours: man and chimp have much morphologically in common so if God designed there bodies to do similar things they'd have similar deficencies.
Again note the qualifier.
And I have to get back to work.