misdiagnosis and malpractice are two very different things.
doctors are human, and they make mistakes; not every (or even most) mistakes are malpractice.
Can't really comment on the rest of it as you're being very general. Will lacking something cause you to feel less healthy? yes. Does that mean that whatever the doc said was wrong with you was untrue? no.
Also, did you tell him/her you were exercising/trying to lose weight and what you were eating? Nutritional needs go up with exercise, and cutting calories may give you nutritional deficits if you are simply cutting calories without watching what you eat. Your doctor won't know unless you mention it.
Most fitness vids tell you to take a multivitamin, anyway (at least all the beachbody ones I've done).
| current weight: 148.5