Vist Security and another called Antivirus Pro 2009 (which hit the scene in 2007 or so) are insidious viruses that virtually no anti-virus product out there can remove. Antivirus Pro spread through flash animation on places like addicting games and actually shut down their website for a while back in 2009. Antivirus Pro embedded itself in the boot sector after a long enough infestation and would therefore become impossible to remove. We lost 2 hard drives to this one a couple of years ago. It is basically a scam to get you to pay them to buy their "product" (Antivirus program) to get rid of the virus they embedded on your computer to begin with. So you send them money, you then download their virus machine to your system and they suppress the evidence for a few months. Then it "comes back" and they recommend you buy the upgrade to their "product" so they can suppress it for a while longer. As long as you pay, I have been told, then it never really causes any damage, but it is designed to get more and more aggressive the more you try to root it out with any other program than theirs. Eventually it hijacks everything (browser, windows explorer, boot files) and the system dies. It can even reach a point where virtually no data is recoverable. It was rumored for a while that it could also break software RAID arrays, thereby wiping data out of connected drives.
Do a search for removal of Antivirus Pro 2009 and follow the steps to give you the best chance to get rid of the Vista Security virus. From what I have heard the Vist Security virus is not as system-pervasive and damaging as Antivirus Pro 2009, but I wouldn't take the chance. It is a very manual process which does involve booting into safe mode, using various antivirus tools (MWB is good) and doing a manual scrub of certain locations to remove the embedded files.
And be afraid. Very afraid.