"Vous devez être un homme."
Vous can be both plural and singular. Plural when addressing a group of people. Or singular when talking to a single person in a formal or respectful way (i.e. people you don't know, older people, etc).
Your example is possible, but a bit strange. Usually you want the numbers to match (plural to plural, singular to singular).
When you have a vowel following a consonant (vous deveZ Etre un homme), you connect the two, carrying the consonant's sound over in what's known as "liaison": http://french.about.com/library/pronunciation/bl-liaisons.htm Hope it helps!