the upcoming release of Vala will have basic scripting capability (mostly for small tests). Yes, this sounds weird, but it's more trivial than what you expect. The installation will create a symbolic link /usr/bin/vala -> /usr/bin/valac that you can use to run vala scripts.
Also, vala scripts can have implicit main(), so that it becomes faster to write test scripts. An example:
#!/usr/bin/env valamessage ("test");
Save it as test.vala, now you can either chmod +x test.vala and run ./test.vala or just vala test.vala.
Valac will automatically compile the script and run it. After that the executable will be deleted, which means there's no cache.
This feature is of course experimental and it's not currently possible to import other vala files.