The Pentax 55-300 has really really good IQ for its price - I do own one and have really got some brilliant shots with it
Compared though to the Nikon 70-300 VR it feels cheap and from 70-200 without a doubt the Nikon is sharp really sharp - it knocked my socks off when I looked at the results
For me in lens vr is much nicer to use with long lenses or zooms at their maximum as it stabilizes the view and helps you concentrate on your subject easier.
I have added a pic here shot at 300mm wide open on South African weaver with zero cropping taken with the Nikon and 3D Continuous tracking and 9 points assist
Weavers dont sit still for a second and are bouncing around continuously especially when I was this close - seems sharp enough for me for a consumer zoom
Although you can't tell, the light was close to dusk and I was shooting at about 1000iso