you have made an interesting question.
First I must say that VPL was not thought to manage large files, the files are fully managed in memory.
In this case, the message "connection fail: error" comes from a PHP Fatal error due to memory exhausted. To avoid that one PHP script uses too much memory (taking memory needed for other scripts) the memory used by each script is limited by configuration (php.ini). The solution to this problem is to increase the PHP memory limit. Notice that binary files are coded as b64 to be sent in JSON.
Due this question, I added "showing a verbose message for this problem" on the TODO list.