Vlingo, a provider of voice-based mobile services, has raised a $20 million second round led by Yahoo, along with participation from past backers Charles River Ventures and Sigma Partners. Last August, the Cambridge, MA-based start-up raised $6.5 million, for a total raise of $26.5 million. The investment from Yahoo (NSDQ: YHOO) is strategic, as the internet company will use Vlingo’s services to voice-enable OneSearch, a feature being unveiled at CTIA. It’s pretty straightforward: rather than having to type a search query, users can speak it. Technically, Vlingo’s service is useful anytime an application requires a text entry box, though search is the most clear application of this. The company says it will use the funding to improve its product and to expand internationally. As mentioned when we previously wrote about Vlingo, dealing with differing accents has been a challenge that the company hopes to address. Release (.pdf)