There’s a small bug in Yosemite that causes the size of the dialog windows to increase in size.
MacVector is affected by this problem.
Basically every time you use a dialog its size increases slightly. Over time the dialog becomes too large to fit on the screen and you will have problems clicking the buttons.
The easy workaround is to hold down SHIFT and resize the window from the edge. The new size will be saved in the preferences when you close MacVector.
Hopefully we’ll see a fix to this in OS X 10.10.2