Google seems to disagree with you...
http://www.google.com/webhp?q=%22vis+in+numeris%22
Google is not able to correctly translate Latin
I've studied Latin in high school (5 years) here in Italy and my wife teaches Latin in high school.
'Vires' means 'soldiers', 'armed forces'.
When used in its plural form 'vires', it may mean 'strength', but only in sentences like 'vires defiunt' which means 'I'm becoming dizzy' (my 'forces' are going away, literally).
If you want to correctly translate 'strength (is) in numbers' the Latin for it is: 'vis in numeris'.
It was even used before, see:
http://articles.latimes.com/2000/dec/17/news/ss-5286
or a manuscript
De numeris: MS Hannover, Landeskirchlichen Archiv D 10 Nr. 1 (PfA Bissendorf HS 20) ff. 186r-195v [inc: Quanta sit vis in numeris noverant...
"How much strength is in numbers ... "
spiccioli.