Question: Maybe this question is better suited for the English Language SE site, but anywho...

V here stands for V-Neck as in: v-neck t-shirt/v-neck blouse/v-neck sweater, etc.

深 is deep - or more accurately here 'low'.

so basically it's a super low-cut v-neck style of clothing - but is there a proper translation for this in English?!

Answer: 
You actually kind of answered your own question: `深V` can be translated as [**"low cut"**](http://www.yandy.com/Low-Cut-Mini-Dress-with-Cinched-Ties-on-Sides.php). Alternative names for that style are [**"v neck"** or **"deep v"**](http://www.missguided.co.uk/dresses/v-neck-dresses) as @Sheng-DuanSun suggested, or the [**"deep plunge"** or **"plunge neck"**](http://www.missguidedus.com/aleena-lace-plunge-neck-puffball-dress).

I believe I've also heard it called the "*plunge v neck*".
The neckline is so deep that almost the chest is exposed.