Surprise - on the Mac you don't need to use a special keyboard layout to type accents. On the plain old QWERTY and Dvorak layouts, you can type an acute accent with option-E. A grave accent is option-`, an umlaut is option-U, a circumflex is option-I, and a tilde is option-N. Rather easy once you realize that most of the option-combos for the accent marks correspond to the letter above which you would often find a particular accent mark, and it frees up the ' and " keys to behave as most people would normally expect (what genius at Microsoft decided that no one would ever start a quotation with the letters A or I, anyway?

).
There are actually a great many special characters you can produce with the Option key. To see what they all are, enable the Keyboard Viewer in the Input Menu.
There is also the U.S. Extended keyboard layout to make more exotic characters such as č, ā, ł, or ț. Unfortunately, I don't think this particular layout has a Dvorak equivalent.