Yinwei or Yinvei? 因为
/v/ is not an initial found in MSM.

The initial /v/, though, is often found in Northern Mandarin and its branches. In fact, it can still be found, even, in distant branches like Sichuanese.
If you ever saw the 马蜂窝 commercial that aired, ever single night, during the World Cup, you'd certainly notice that 唐僧's &quot;为&quot; is very /v/'d.
You can also refer to the paper The geographic distribution rules and reasons of zero initial “w” pronounced as “v” in Mandarin  by Wang Fan. The abstract for the paper goes into some detail in English:

Abstract:
It  is  universal  to  pronounce  zero  initial  “w”  as  “v”  in Mandarin. Researches  show  that  38%  of  the  people with northern dialect do so with 10% in other dialect districts. When analyzing its features and reasons, we find that the dental fricative sound “v” is indeed a variant of the semi-vowel “w”. Its coming into being is affected by the variation of the  mouth  in  different  phonemes.  It  is  also  related  to  individual  pronunciation  habits  and  the  instability  of  the  pronunciation.  The  universal  “v”  pronunciation  in  northern  dialect  district  is  caused  by  the  stronger  movement  of  the mouth and the face in pronouncing than that in other dialect districts.


The 为 in 因为 should be pronounced wei if you're going for &quot;standard&quot; Mandarin. But, if you want to make yourself sound more Northernized you can also opt for the v.