I don’t know, and I wouldn’t be surprised if no one else does either, given the difficulty of measuring low-level irreversible damage.
A vegan will presumably / hopefully be aware of the dangers so they would probably catch it before much damage was done, but I wouldn’t be confident that NO irreversible damage would accompany symptom onset.
If you are a random case, the neurological symptoms can be hard to interpret and won’t necessarily result in a B12 deficiency diagnosis, at least not at NCKU hospital. I had 4 different neurologists telling me 3 different things, but the take-home message was that I had an incurable but slowly deteriorating condition.
In fact it now appears I had an easily curable but rapidly deteriorating condition, and the latter fact became obvious to me quite quickly.
If I hadn’t spent a solid week researching it (mostly wasted on inconsistently applied and recorded nerve conduction test results, which were basically uninterpretable), worked out my own program of diagnostic tests (which included vitamin levels, though that wasn’t their focus) based on best practice from the literature, and insisted on them doing them, I’d probably be dead.
As it was I almost certainly suffered some irreversible nerve damage (NCV results remain lower than normal several years later) but no one has been very keen on quantifying that.
Left to their own devices they were going to piss about with genetic testing, a completely pointless activity from my point of view, and quite expensive.