Can just taking a multivitamin reduce the symptoms of vitamin B12 deficiency?

It depends... This depends on the cause of the deficiency. If it's due to a lack of B12 in your diet (b12 is primarily found in animal-derived foods) then taking a multivit. With B12 will help. However, many cases of B12 deficiency are caused by difficulty absorbing it, a condition called pernicious anemia. In that case, oral B12 won't help & you need injections &/or sublingual b12, ideally as methylcobalamin.