1.)Can a VFR pilot use ILS for landing?
Yes, as additional aide for visual landing.
2.)Can a IFR pilot fly as a VFR pilot?
Yes. My ATP certificate permits flight under either Instrument Flight Rules (IFR) or Visual Flight Rules (VFR). Just not both at the same time.
FWIW, it is not uncommon to operate a single flight using both sets of rules (IFR and VFR) multiple times during the same flight. The last Dutchess flight I took my family on started out VFR in Nevada, changed to IFR north of LAX area, changed to VFR near SNA, finished IFR approach/landing in San Diego. Flight plan was a VFR flight plan based upon forecast wx.
3.)Is a VFR plane different from a IFR plane(like instruments.........)
If you define "VFR plane" to mean "VFR only plane", then yes they are different. A "VFR only plane" lacks some or all instruments required to be certified as an "IFR plane."