How do I close out a spread (vertical) in Thinkorswim with a single order?

You can use think or swim's interface to close out a spread position.  Bring up the monitor tab and find the spread you want to exit.  Click on each leg of the spread (use CTRL+LeftClick to select multiple legs), and once they're highlighted, RIGHT-click on any of the legs, and select a closing order from the popups that appear.  E.g., if you sold a vertical, then you would select the choice to buy it back.  Once you select one of the offered choices, the order will appear in an order window and you can adjust the parameters, such as limit, trailing stop, etc.  The following image shows a sold call vertical's buy-back choices, where you could buy back the whole spread, or either leg individually (for example, you could buy back just the short leg if desired).  As with anything new, make sure to practice this on paper to get some experience with it before trading with it for real.