I realized that the value of vacuum_buffer_usage_limit parameter in postgresql.conf.sample doesn't have the unit:
#vacuum_buffer_usage_limit = 256 # size of vacuum and analyze buffer access strategy ring. # 0 to disable vacuum buffer access strategy # range 128kB to 16GB
It works but I think we might want to add the unit kB for understandability and consistency with other GUC_UNIT_KB parameters. I've attached a small patch that adds the unit and aligns the indent of the comments to the perimeter parameters.
I'm not currently able to check, but if work_mem has a unit in the sample conf then I agree that vacuum_buffer_usage_limit should too.
I'm fine for you to go ahead and adjust this, otherwise it'll be Monday before I can.