Welcome Andrea,
The Vne of a PA-28 will vary by model, so you'll have to specify which PA-28. To help yourself, you can look it up, as well as lots of other fascinating information about any certified aircraft, in its Type certificate Data Sheet:
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/58e3800616d183568625805f00534b82/$FILE/2A13_Rev_56.pdf
The approach speeds will not be described there, as they are not "limiting" to the type. Expect that to be a range, spanning 10 knots or so. Precision to within a knot or two is not required for approach speed of a PA 28.