Yes, they are capable of manufacturing some of their own
vitamin C. But that is not to say they should not be fed
food rich in vitamin C to ensure maximum intake of this
most essential and non-toxic vitamin.
Vit C like vit B will not be stored in the body and any not
required at the time will as Jeangenie correctly says
be "excreted in their urine".
The link Jeangenie has posted above is a little confusing,
it states "vitamin C is not "natural" for a dog's dietary intake"
However a dog in its "natural" habitat would devour the organ
meat from its prey, a "nutural" source of good
amounts of vit C