1) No, I run Skype of Win7, WinXP, the Ipad, and the Ipod. No issues.
2) I can't see why it would but I can't say for certain. I use VOIP (but not Vonage) and don't have a problem.
3) I hear people like the Apple product but Skype is pretty good. I think it's probably more a matter of taste.
4) I doubt it but it's easy enough to install and test.