If you change the backside of your card to this, it should hide the vocab translation only if there is something in the J-Definition field.
<div class='jpsentence furigana'>{{furigana:Expression}}
<br/>
{{^J-Definition}}<span class=vocabkana>{{Vocab-Kana}} </span>
<span class=vocabtrans> → {{Vocab-Translation}} </span>{{/J-Definition}}
{{#J-Definition}}<div class=jdef>{{J-Definition}}</div>{{/J-Definition}}
{{#Part-of-Speech}}<span class=pos>[{{Part-of-Speech}}] {{/Part-of-Speech}}<span class=notesprod>{{Notes-Vocab-in-Expression}}</span></span>
<div class=ensentence>{{Sentence-Translation}}</div>
<div>{{Image}}</div>
{{#Kanji-WK}}<div><strong>Kanji-WK:</strong> {{Kanji-WK}} {{/Kanji-WK}}
{{Audio}}
{{Sentence-Audio}}
<style>
b { background-color:transparent }
.nofurigana ruby rt { opacity:1;}
</style>
By the way, I only learned about this Anki feature a few days ago - after I followed your guide for Subs2SRS and Morphman! Thanks for the guide!