First of all how do you know this is Jerome's version? The new Vulgate (in Latin) is not the Jerome's original.
Besides, I don't know what version of Isaiah 9 Jerome use for his translation, considering that he did not write the OT based on the LXX but on the Masoretic Text (MT), and most Jewish versions of the English Hebrew Bible quote Isaiah 9:5 as
Whatever that name is, the NKJV is not following the LXX or the MT.
” Pele-joez-el-gibbor-Abi-ad-sar-shalom” is not a name, it’s just left untranslated for whatever reason.