I made a demon hunter saved up my currency and purchased the leather sets from the vendor, I think it was the rouge one expecting them to show a complete set. nothing, I did get the transmogs I was missing but I didn’t get an option to view the full set I purchased. Later I purchased the druid set yet again no set appeared in my transmog window I did get the missing appearances in induvial slots windows.
Maybe make the transmog set like the one you can purchase from the darkmoon faire, I know from personal experience the dungeon sets from classic you can buy show up in the tab for those who can use them of the right armor type.
As a side note can you make the currency need to buy complete sets less if you have some of the appearances or just let us buy missing transmog items we need.