There is definitely some overlap in what Vox is doing and what Google wants to do as well. Here is a good list of the commands, and I understand that some of these commands already have other functions in Vox's default commands, at least its XBMC integration component.
http://forum.xda-developers.com/showthread.php?t=1961636
I can't help you with your dream of integrating Google Now I'm afraid, but this afternoon I tried most of the example voice commands listed in your xda forum link with minor adaptation of my existing custom commands and had pretty good luck.
To be fair, I already use the weather plugin (WUnder), the Google Calendar plugin etc., so it didn't take much doing. The Wolfram command (I've updated it a bit) actually works for a great many of their examples -- calculations, stock quotes, distances between cities, authors of books, height of Kobe Bryant, who's married to whom, ages of famous people, the weather on Friday, etc.
I used slightly more natural phrasing for many of my searches than they give in their examples, but I imagine natural phrasing might work OK with Google Now too.
To pull up a Google map is pretty easy with a Launch.URL command. An "I'm feeling lucky" Google search is also a good backup command.
As you say, we can already do a lot of things like opening/closing programs and websites, checking our google calendars, email and so on. So the main things that would take a bit of creativity would be navigation commands and that kind of thing.
I've attached a group of commands that you can play with a bit and adapt to your own needs/voice command phrase preferences if you like. They're somewhat customized to my metric locale. (The weather command requires enabling and configuring the WUnder plugin.)
These successfully generated useful responses to about 90% of the xda example voice commands, but only when I'm using my "good mic", on account of the open dictation. I haven't included Google calendar-based commands, but I think there are probably some examples here on the forum if you haven't tried those out yet.
Welcome to the forum.
[anyone trying the attached Wolfram command will need to use their own API key in the command, of course.]