thanks to jeev on ##slackware on freenode, this is now working.
I was missing
from my vsftpd.conf. See line 132 from current/working vsftpd.conf file on pastebin
please note that I also edited from the original:
write_enable=YES (line 18)
Once I get some time over the next couple days/weeks, I will provide a full how-to for Explicit FTP over TLS using vsftpd on Slackware64 13.37