Is Ventnor (Isle of Wight) the largest UK town beginning with the letter V

Not based on population. According to 2010 figures, Ventnor's population was 5,980. Vale on the island of Guernsey has a larger population of 9,573. Verwood in Dorset is the big daddy of UK towns starting with 'V' however. It has a population of 14,820 according to 2009 figures.

I didn't count Vauxhall or Victoria since those are London boroughs.

Verwood in the New Forest is about twice as large. Of course, if you counted Vauxhall in London...