The reason that neuros wait for a second attack to diagnose relapsing remitting MS (RRMS) is that there are actually quite a lot of things that can cause a one off attack (e.g., ADEM). So, while your symptoms are very MS-like and you had that lesion in your brain, there is actually a good chance that you will never have another attack and therefore never develop MS.
A lot of people find the “when/if it will happen” worry very difficult to live with, but I’m afraid getting a diagnosis doesn’t change anything: no one with RRMS can predict when their next attack will happen or how long it will last. Uncertainty is a huge part of living with RRMS Obviously, having an MS diagnosis makes symptoms a lot easier to explain to others so, if this is one of your concerns, you could ask your GP for the proper term for your condition, e.g., CIS, clinically isolated syndrome (basically a single, MS-like, neurological attack).
Vitamin B12 deficiency can cause neurological symptoms, but you shouldn’t get any new ones once you have your levels under control. Telling the difference between B12-induced symptoms and CIS-induced symptoms is very difficult though so I’m afraid you’ll probably never be able to resolve that one. If you get new symptoms and your B12 is fine, then it’s definitely neither and you need to see the neuro again.
It might be worthwhile taking a vitamin D3 supplement as well as the B12 injections because low vitamin D has been linked to the development of MS and lower levels of vitamin D are associated with relapses and with more severe MS. Most neuros are now recommending that people with MS take a vitamin D supplement although the amount varies. The most common amount seems to be 5,000iu a day. You can buy these on-line at a very reasonable price (~£12 for a year’s worth). Your attack may not be going to develop into MS, but there seems no harm in stacking the odds at little more in your favour?