Is a heart rate of 134 beats per minute too much for a very active 5 year old boy who has been taking viral choice vitamins in the morning?

If that's his resting heart rate, you can have him checked by a doctor and see if he has some heart problem. If that's his heart rate while active -- not after resting for five minutes -- then it's within the normal range. I notice that viral choice vitamins are not recommended for children under 10. Perhaps you should discuss this choice with your health care provider, if you haven't already.