Welcome to the network, Jim!-- R

I would say that in the way many of us use the terms, a vihuela (predominantly 16th century instrument with typically 6 courses) _is_ a different instrument than the guitar (4 courses in the 16th century, and 5 courses with variable tuning from the 17th century to the mid-18th century). However, if you are talking _only_ about the 17th century, then term viguela or biguela or other variations on "vihuela" referred to a 5-course guitar.

I would suggest you begin a forum topic here with your questions.
You might also consider joining the Vihuela and Baroque Guitar email list. Though you would get emails on all the subjects discussed, you would reach some people not connected to this network. You can find information on joining the email list at:
http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html

You might also want to mention what time period you are interested in, what countries, court or popular, etc -- any details that might help better focus responders on your specific story needs. -- R