As long as they are in their original container, you should have no problems with vitamins. Bike tourists rarely get checked at the border unless you get a red light when crossing. If you do, the search is generally minimal. Be prepared, however, for sporadic military checkpoints in all of Mexico's northern territory, all the way to Nayarit. If you're going through Baja, I've heard you'll have a pretty thorough customs and immigration inspection in La Paz just before boarding the ferry to mainland. BTW, don't forget to get your visa (at your first entry city) and keep the "paid" receipt at all times. You can only get a tourist visa at the border which costs $26. If you lose it you'll be forced to get USD $140 FM3 visa.
Last edited by Chris Pringle; 08-28-12 at 08:06 AM.