That's because there isn't one. However, that doesn't mean there isn't a solution to your problem. While you can't load a new VSP, per se, you *can* access the VSP as an image and copy stuff over it.

The handle for the image containing the current map's VSP is curmap.tileset. The image size is 16 x 16 multiplied by however many tiles there are in your VPS: essentially a long strip of tiles. It would be no problem to create a seperate set of tiles as a similarly formatted image file and simply load it into memory and then simply blit it over the top of the current tileset.

The fact you can access the tileset in this way even allows you to (theoretically) produce procedural animations in your VSP: little graphical effects that are controlled via code rather than drawn by hand, like smoke or leaves that sway dynamically in the breeze. Might be a bit of a performance hit if you really go to town, though. Something to think about if you want to save some time on producing art. Just think: sod drawing all those trees, write an algorythm to produce them at runtime.