How far apart do separate vessels have to be to be considered different processes?

There is no hard and fast rule for how great this distance should be before you do not need to consider the vessels as part of one process.  Two vessels at opposite ends of a large warehouse room might have to be considered as one process if the entire warehouse or room could be engulfed in a fire.  Two vessels separated by the same distance out of doors might be far enough apart that a fire affecting one would be unlikely to spread to the other.  You may want to consult with your local fire department.  You should then use your best professional judgment.  Ask yourself how much of the regulated substance could be released if the worst happens (you have a major fire, an explosion, a natural disaster).