By Pamela Barbaglia and Andres Gonzalez LONDON (Reuters) – Vodafone is speeding up talks with a handful of financial investors to sell a slice of its wireless towers unit as it hopes to agree a deal by Nov. 15 in conjunction with its half-year earnings, two sources close to the matter told Reuters. The British telecoms group is evaluating proposals to either halve its nearly 82% holding in Frankfurt-listed Vantage Towers or just sell 25% of it in a bid to free up cash and respond to activists’ pressures to shake up the company and improve profitability, the sources said, speaking on condition …