Most reports say that turning off Vista's default "Aero" theming system helps a lot here. You might start by changing the Appearance to "Windows Classic."
I suspect there is a better workaround than this but I don't know what it is. Has anyone tried an application manifest for the IDE with the Common Controls 6.0 <dependency> node yet?
I believe another way to go is to disable Aero just while you're working in the VB6 IDE. Right-click on VB6.EXE and select Properties I think, then in the Compatability tab be sure to check "Disable desktop composition."
I could be more certain if I had a Vista machine in front of me, but I don't right now. This should be close if not correct though.