No. There is no reason to have any vitamins given IV unless in some cases when you are hospitalized for certain conditions. For certain vitamin deficiencies (usually b12) we sometimes administer vitamin into the muscle or the skin by injection. Vitamin d for example is fine to take orally if needed.
No. I can't think of an instance where i'd recommend an IV vitamin outside of something like iron in someone who just can't absorb it. Don't let a non-doctor push a treatment such as this on you. Vitamins can be toxic in high doses.
Not necessarily. If a patient is fatigued i'v bypasses the liver one time helping you absorb vitamins more completely.