Most surgeons provide their patients with a complete list of foods and medications to avoid prior to surgery. The most common of these are aspirin and aspirin-like products like Motrin, Advil, Aleve, etc. In addition, common supplements to avoid include Vitamin E, glucosamine/chondroitin, and fish oil. Certain foods can be rich in Vitamin E and natural salicylates and should be avoided. In general Magnesium is safe, but still would be wise to check a magnesium level as part of your routine preoperative bloodwork. Vitamin B12 is water soluble and is hard to absorb from your GI tract so it is hard to get a toxic dose of B12. Even toxic doses with intramuscular administration are rare. But still would be prudent check with your hematologist to assess your dose of Vitamin B12 and your need to be on it. Good luck with your surgery.
I usually ask my patients to stop all vitamin supplements and herbal remedies at least one week prior their surgery. Simple ginger and at times green tea can have the effect of blood thinners in certain patients.
Arnica and Bromelin before surgery are proven to reduce bruising and swelling.
There is no issues with any vitamins & general anaesthesia. However some increase bleeding after surgery but vit B12 & magnesium don't normally interfer with this.