Currently, the 0.5 inch Vickers has no penetration value. Apparently, this weapon could pen at least 15mm at 400 yds. per RN requirements.
In 1928 the RN studied and compared the 0.5 inch Vickers and the 50 cal Browning, resulting in a contract with Vickers. The Vickers was considered much more reliable.
There is a suggestion that the Browning could be slightly more powerful, and I believe the Browning had a longer barrel so was probably more accurate. Currently the game gives the Browning an accuracy of 23 and the Vickers 14.
Here’s the source:
http://www.quarry.nildram.co.uk/Vickers.html
Not sure how the Army AFV mounted version differed?
This isn't a big deal, but thought I'd mention it in case you guys would like to edit this weapon for a future update. In fact, the Matilda I (unit 027) is the only unit I know of that uses this weapon.
Here's another link showing a pen. of 11mm at 91M and 9mm at 457M:
http://gva.freeweb.hu/weapons/british_guns1.html
cheers
Cross