[vaí] post positive pcl. emphasizing the preceding word: rare in the Samhitâs, very common in Brâhmana (often indicating the beginning of a clause) and Epic; in Sûtra generally only in the combination yadi ͜ u vai; in Epic and Manu vai is very common at the end of a pâda as a mere expletive; with other pcls.: vâ u (Rig-veda); ha vaí, ha sma vai, eva vai (Brâhmana); api vai, tu vai (Classical (post-Vedic) Sanskrit).
Source: Macdonell