I doubt if VEX is an upgrade of A12t. To me both have similar technicalities, just different in tuning. 12t is mild U-shaped, while VEX is warm neutral. VEX sounds more relaxed and has better bass slam, while A12t presents music in a more 'organized' way.
And Phonix sounds pretty different from VEX in fact. I personally find Phonix to be a thicker VE8 with better technicals, while VEX sounds like a more musical VE7 with upgraded techs. Comparing the Phonix and VEX Phonix has thicker lower mids, which sounds more organic. So I would suggest you better demo VEX before your purchase. They still have the VE DNA though, so you won't go wrong with either one after all.