When you use Vonage does it slow down your internet connection such as cable?

All VoIP services use a portion of your Internet bandwidth. How much depends on the Internet speed you are paying for... 256Kbs, 1Mbs, 3Mbs with your ISP. With 1Mbs speed or faster, you should not notice any problems with your VoIP quality..... unless you are downloading big files on your PC while talking on the VoIP phone.... in which case voice quality may start to break-up. I have 256/128 Kbs Lite speed from my Cable ISP and currently use VoIP exclusively for my phone calling. I have no problems with voice quality.... as long as I don't surf the net or download files while on the phone.... in which case my voice audio will break-up a bit. This is just a function of Internet bandwidth. Someday, when saving money isn't such a big issue for me, I will upgrade to 1Mbs or 5Mbs service....

As the first answer said, it has some effect, but not enough to be bothersome. I have Vonage, actually have had if for a year now, with no major problems( It's cheap too)

Yes, it will use some of the Internet bandwidth, so it will slow down the Internet some, how noticeable this is depends on how fast your Internet was to began with.