I've got Macs and PC's on FiOS at home and don't see a difference like that. Often we get close to my advertised download speed on big files.
You didn't mention what kind of Vista machine you have (desktop or laptop) but the Mac is a laptop. Are you maybe testing wired vs. wireless? My network speed starts to degrade pretty quickly as the laptop moves farther from the wireless router.
Verizon FIOS has a tool to adjust some settings on a mac - if its laptop it will effect off site use so then you should uninstall it. My macs on with 100% 30down/5up always worked as long as it wasn't doing a download or sharing some files somehow.