sounds like a pretty common VX problem, if you do a search for 'rough idle' im sure you will turn up lots of results, if you take your VX to an autozone, they usually have the diagnostics tools you can use to get the code and clear it if youd like, once you get the code type it into google and see what it means good luck!

p.s. i have a tool that reads/clears codes (scangauge II from tone, id recommend buying it if you have the means) and am not that far from you, if you cant find anywhere to do it for free let me know