Latin, like pretty much all other languages, went through an evolution in its pronunciation over the years it was spoken. In early Latin, the letter which was written as a V would have had a sound like a modern English W. As the years went by, this sound gradually changed to that of the modern English V. But when Julius Caesar said "Veni, vidi, vici", it would have sounded like "waynee, weedee , weekee ".

As an example, the Latin word for wine is "vinum" and in Caesar's time would have been pronounced "weenoom". It was at this time when Roman traders begin to interact with northern Germanic tribesmen and introduced them to the beverage made from fermented grapes. The Germans adopted the Latin word for this beverage for their own, and it was eventually passed into English as wine, with the initial W pronunciation.

Meanwhile, in the Roman empire, Latin pronunciation continued to evolve. By the time of the late empire, the word for wine would have been pronounced with more of a modern V sound and would have been pronounced "veenoom". As Latin eventually evolved into Italian and French, for instance, those languages inherited the newer pronunciation with a V. The Italian word for wine is "vino" and the French is "vin", both with the same sound as the English V.

Other letters changed their pronunciation as well. The letter C, for example, started to pick up more of a CH sound in later Latin. Thus, someone reading the works of Julius Caesar in the late years of the empire would have pronounced his famous declaration as something more like "vaynee, veedee, veechee". This CH sound for the letter C was passed along into modern Italian.

Latin itself stopped evolving once it was no longer being actively spoken as anyone's native language. It remained in use, however by the Church and as a language of learning. Church Latin tended to use the later pronunciations, including the more modern V sound.

All of this is of course a fairly roundabout way of saying that when the Crusaders gave their famous cry, it would have sounded like Deus Vult rather than Deus Wult.