What does 'Vision upon Vision' mean?

More context is required to understand the full situation at hand, but I'd say with high probability that it means, "one vision after another", or "multiple visions in succession".

Since you're talking about a "vision" (which isn't something physically tangible), it wouldn't make sense to interpret the phrase as "a vision on top of another vision", although, that interpretation could still be correct if – say – your event is taking place in a dream (where anything could happen).