Don't beat yourself up about being a vegetarian and possibly doing this to yourself. How would you know to look for it unless your doctor has been testing you for it. And as we age, we don't absorb our nutrients as well as we used to.

I assume they have checked your iron stores, vitamin D and copper.

For now, I wouldn't worry about additional testing for B12. But for future reference, looking at homocysteine levels and MMA in the blood would be the next step if you suspect a B12 metabolism issues.

If needed, restoring B12 is usually pretty easily done with oral supplements. I would first start out with the Methyl form of B12. I prefer to start to start low and go slow to see how you do. Some find B12 stimulating while others find the opposite so pay attention how you feel. If it's too stimulating, then take it in the morning. Jarrow makes a good sublingual at 1000mcg or 5000mcg. If you do ok with B12, then you should add in a good bio-available B complex after about a week or so. Jarrow also makes a good formula. The B6 and folate in it are in a good form.

Spend some time looking at the B12 stickie at the top of the PN forum. There's a great video about one doctor's story with B12 deficiency that's worth watching
Diagnosing and Treating Vitamin B12 Deficiency - YouTube