Have a technical question you can't find the answer to in FAQ's? Ask it here. Game problem questions that do NOT pertain to things like training advice, what would my horse do well in, how much is it worth.
If you go to the vet skills page, select the vaccinate option and choose the horse you want to vaccinate from the drop down list. Then click the "proceed" button. Make sure you've bough the vaccines first. You can buy them on this page: http://www.virtualhorseranch.com/items/market.php