A network for historic guitars and vihuelas
It must have a sound hole of some sort. I am no expert but my vihuela has the sound hole cut directly into the table in a geometrical pattern.
My baroque guitar - which is a much nicer instrument - has an elaborate parchment rose built up in layers. Just looking through my collection of CDs of various luminaries playing the vihuela, a simpler rose seems to be de rigeur although in some cases this may be cut flat out of parchment rather than cut into the table. A sotr of cross between the lute and baroque guitar.
Many vihuelas have a rosette of which some are made of cut and glued parchment and some are carved from wood.
The black stuff you see may be just the shadow cast because not much light hits the inside of the instrument as compared to the outside, which then translated through a typical camera's dynamic range, won't show many details on the inside ;)
If you don't feel like cutting a rosette, you may leave a hole, which is depicted in some paintings with a vihuela.
Go to the site of the Jacquemart Andree collection in Paris and look for the vihuela there. It represents the only indisputable example of a sixteenth century vihuela (albeit a bass size instrument). Obtain a copy of the plans and base your reconstruction on this - necessarily scaled down if you require an instrument around nominal G tuning rather than a 'bass' vihuela
Here's one link:
You don't need to construct your instrument with the elaborate 'jig-saw' make-up of the original. You'll also see that the (original) rose design is much plainer than found on most later five course guitar roses. And perhaps just have a single rose (although in the same pattern) rather than the original which has one large central rose and four satellites (interestingly, all with a similar design).