I see nothing related to Vista on the supported system requirements page. Call product support. Before you make such changes in the future to an OS or browser which was not available for testing when LoadRunner (or QTP) was under development, consider confirming the level of available support with HP|Mercury CSO prior to making the switch.
A check of mercury's support site also states that there is an IE7 patch for many of their products. The link can be accessed from the support homepage (IE7 info is about 1/2 way down the list of alerts).
I also didn't see anything there regarding Vista. As James stated, you should always check before doing an upgrade.