For both instruments the two parts of the bottom have been joint.
In this picture - nearly - all the parts of the viol.
In the foreground the 'false bottom' on which the belly is to be built up.
The neck is sawn from a piece op maple (a leftover from earlier instruments).

You can see the first impression of the form of the vihuela in this picture.
This instrument won't get a top block, but the sides will be joint in the neck.