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.
Khandhānaṃ rāsaṭṭhaṃ, āyatanānaṃ āyatanaṭṭhaṃ,
Dhātūnaṃ suññaṭṭhaṃ, indriyānaṃ adhipatiyaṭṭhaṃ,
Saccānaṃ tathaṭṭhaṃ aviditaṃ karotītipi ‘avijjā’.
It prevents knowing the meaning of heap in the aggregates, the meaning of actuating in the sense-bases, the meaning of voidness in the elements, the meaning of predominance in the faculties, and the meaning of suchness in the truths, thus it is called ‘ignorance’.
(Visuddhimagga XVII. 43)