My web site is growing and in order to limit my upload time and to provide ease of maintenance, I would like to put pages in subdirectories. Are there any ways to put different pages in different subdirectories? Thanks.

This may be possible.If you use the "Additional Files>Add Files" under the settings, and make a new folder, name it and upload through VSD. I haven't tried it myself yet, but I did do a quick test offline.
NOTE: keep in mind the VSd makes it's own folder for the images,
but for anything that you need to use the additional files wizard for may be placed in a new folder. I wil do more testing .
UPDATED: Scotts right!

Unfortunately there is no way to do that. With Bruce's idea that would only work with pages created outside of VSD, like something from Photo Gallery etc.