Mixing vitamins is perfectly acceptable as long as they complement one another. As explained by ConsumerLab.com, certain vitamins, such as vitamin C, actually increase the absorption of other vitamins.
To be safe, ask a pharmacist or other health professional before taking vitamins together. Mention all medications that you are taking, as some vitamins and minerals interfere with the effectiveness of certain medicines. Always read the directions on packaging, and refer to the recommended daily allowances when deciding on dosage. When in doubt, always err on the side of caution. Finally, take note of special instructions, such as "take with food," as some vitamins and supplements can affect the digestive system.