What is the smallest scale at which blood vessels, nerves and other structures are deterministic?
Indeed a very good question. I'm afraid it might remain without a proper anatomy-based answer, but my intuition would tend towards agreeing with "the smaller you get, the more deviation you'll find".

Or rather, I would expect the same principle as in conservation of genetic patterns to apply here: the more central a tissue structure is to survival, i.e. the more downstream systems depend on it, the more likely it will grow in conserved patterns.

As an excellent example, the largest blood vessels including the aorta and its first- and second-degree branches are identical in branch number, approximate relative branch size and approximate branching location across probably all living humans (given that babies born with malformations on this scale would probably not survive without drastic surgery). Similarly, the largest-level nerves and nuclei are identical across the species. These are central systems that provide core functionality for survival.

On the other hand, teeth are quite important, but the odd tooth missing or additional doesn't harm terribly as no downstream systems depend on them. Indeed, it isn't uncommon for humans to be missing a permanent tooth and keep their primary (baby) tooth in that spot for decades; similarly, missing 8th teeth or additional 9th teeth also occur.