For all-intents-and-purposes, Vertical Leg Press Calf Raises are an upside-down Donkey Calf Raise. Either exercise - Vertical Leg Press Calf Raises or Donkey Calf Raises - is a great choice for performing when your lower back's fatigued from a heavy-squat session. I'm aware many gyms will have neither a Vertical Leg Press or a Donkey Calf Raises, however, you can still perform the exercise. Remember the famous image of Franco Columbo on Arnold's back? If so, Arnold's performing Donkey Calf Raises. Okay, not my ideal choice either (to have someone sat on your back - particularly if your training partner's a little on the heavy side), so give this one a go instead...
Smith Machine Donkey Calf Raises (note: remember to adjust the smith machine safety stops!). You can find Smith Machine Donkey Calf Raises in our Exercise Index.