I suggest you start slow and work up to where you think you should be. Every one's system is different and some herbs are more potent than others. Yes, you can, just use good judgement. We are all different in our body make up. Results depend on how consistantly you take your herbs.
Many people see results in a week.
The longer you have had the problem, the longer you may have to wait to see results. A good rule is to continue taking herbs in moderation, throughout your life, not only when feeling bad, but also as a preventative measure. Herbs should be stored in airtight containers away from sunlight and in a cool dry area. Yes, vitamins and herbs work well together and often compliment each other.