Hi
"........but I want an option that it always chooses to save in the same directory as the source file. ...."
Your particular way of working would seem to suggest that your source files are all in the same folder. That's great and in my opinion it's probably the proper way to go and it's tidy. An option like you suggest could be made to work if that was always the case. However I'm pretty sure many users have source files for a video they are creating in different folders, perhaps because they need to actually keep them separate.(Once selected and in the bin, VP knows where they have come from) In these situations which would be the source folder for VP to export to?
As mentioned you can set a default folder. Now...If you always use this for your source files at the start that's where your export will go automatically. It would be a simple matter to Cut/Past the contents to a dedicated video folder when all is finished so leaving the default folder empty as the "new" source folder. There would be no need to change the name each time. It would automatically come up in the box. (See Borate's post above)
In fact VP already does something like this when you save your work as a Portable Project. In this case the source files are copied into a numbered/dated folder along with a vpj file.
It ought to be possible to do something like this by having an option that exports the finished video along with the copied constituent files (plus a vpj as well possibly) A bit of an overkill perhaps but at least everything plus the end product would be in one place automatically even if the original clips were all sprinkled everywhere.
Nat