is it possible to tell vsd to save the pages as .php instead of the default .html

Sorry, but no. VSD will only create HTML pages.

thank you,
I'll just change them manually.

Keep in mind that any manual changes (outside of the program) will be over-written, and thus need to be re-done, should you update the site via VSD and re-publish.

You 'might' at this point, consider adding HTML Editor to your toolbox (if not already there) and make all future changes through it rather than VSD. I know there are some here that use VSD to get the site up initially, then tweak and update with HTML E. I tried to go that route, but am not visually talented (?) and prefer to use a template as a starting point, and manually manipulate everything. And I barely even dare to tweak with php as of yet, played a bit, but not anything serious.