uclust exit code 137 usually indicates that your system ran out of memory (during execution of assign_taxonomy.py). How much memory do you have on your host system, and how much memory did you allocate to the VirtualBox? If possible, try increasing the amount of memory allocated to the VirtualBox and rerun the command.
Unrelated note: your otu_settings.txt file has:
pick_otu: enable_rev_strand_match True
If you want this option enabled, change it to:
Note that this will roughly double the amount of required memory.