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: 
Well... 深V is not a formal Chinese word, this word was created by bra-producers.
Who are trying to emphasis that their bra can help you to squeeze a **cleavage**.

After that, some dress producers who are also making those dresses with very low V-neck revealing females cleavages say that their dresses are 深V too.

Anyway, in most cases, 深V is the word describing clothes that are able to show the female's chest or cleavage.

深V is female only, if you want to describe male's V-neck cloths, just use V領. Which means V-shape collar.
The neckline is so deep that almost the chest is exposed.