Since you mentioned vim, I suppose you are running yEd on Linux.
In this case, yEd first tries to open the file using the Java Desktop API. If your system supports Java's browse action, that action is used. Which browser is launched depends on your system configuration (i.e. your desktop's associations) in this case. If Java's browse action is not supported, yEd tries the following commands (in the given order) to open the file: