Are you looking for a way to use vials and whatnot in battle?
Try pressing the 'I' key (default for bag if you didn't change it) or click the bag icon in the lower right hand corner of the screen.
When your inventory opens, look for the item you wish to set, and much like you'd set a new skill, hold the left click on your mouse and drag the item to the hotkey button you want to bind it to.
Do note that, as far as I can tell, the top row doesn't naturally have keyboard shortcuts on it, so use the first two rows for anything you want in battle, and the top for out of battle stuff to right click on, like sitting. (thats just my recommendation, to each their own, after all)
Oh, if an item or skill is already on the hotbar somewhere, and you want to move it, simply do the click and drag method on that icon from one slot to another, there's no need to have the inventory or skill menus open.
If anyone notices anything I missed, or simply wants to correct any misinformation I have, please add it.