Every text on Latin I've read says that 'V' is pronounced as an English 'W', yet in 'in vino veritas' it's always pronounced with an English 'V'. Which is correct?

Answer 1

In classical Latin - i.e. the language spoken by the Romans during the republic and empire - "v" is pronounced like "w."

In ecclesiastical Latin - i.e. the version of Latin used by the Catholic Church and spoken throughout the middle ages - the "v" is pronounced in the manner that you are accustomed to.

Both are "correct" insofar as it depends on factors like personal preference, whether one is a classicist or medievalist, etc.