Bug: block processor + nested `read` leads to exponentially long and redundant `vp_sources`

This is due to sources being copied to the temp doc inside of block processor, and then merged back to the outer doc, yielding an exponential number of (redundant) sources listed and slow behaviour (~1-2min after 18 files).

Possible solutions:
- change vp_sources from tuple to set
- avoid redundant source adding in block processors