Side effects of becoming vegan or vegetarian include feeling healthier and happier!
Vegetarians do not have problems getting protein, calcium, iron and other vitamins. You don’t need vitamin supplement; you’ll get all the calcium and iron you need from eating dark, leafy greens (Spinach, kale, collard and mustard greens. Super easy!)
The one and only vitamin that is of concern is B12, find only in animal sources. Your body only needs it in small amounts, plus it stores it for up to 5 years, so you could go vegan and not eat an animal product for up to 5 years and be fine. So no serious need for supplements there either.
Keep in mind that putting too many vitamins in your body actually causes cancer, and that some vitamins help protect not only healthy cells but cancerous ones as well, so you’d just be hurting yourself. (http://well.blogs.nytimes.com/2008/11/20/news-keeps-getting-worse-for-vitamins/)
Also keep in mind that, even though you’re not eating meat, you’re still eating animal products and therefore you’re eating whatever was in the animal. The vast majority of farmers pump antibiotics and hormones in to animals to make them grow, or produce more eggs/milk. So, be careful where you are getting your eggs and dairy.
Good luck on your vegetarian journey. Eat properly, and you will live a long, happy life!