The classical Pali grammars classify the consonant as dento-labial (i.e. requiring the upper front teeth to be in contact with the lower lip). So, v is more likely the correct pronunciation, since w is a bilabial consonant. But in practice SE Asian Buddhists will pronounce it as a w because their native tongues (Thai, Lao, Khmer etc.) don't have a v sound.
The Great Sage did thus proclaim
With many a diverse chosen name
Vipassanā stilled and purified,
That to Emergence is allied.
Saṃsāra-cycle’s swamp of pain
Is vast and terrible; a man
Wisely should strive as best he can,
If he would this Emergence gain.