This is counter intuitive, after all the retina display is the ultimate in resolution no?
However it makes sense when you think about it as the machine doing the rendering of the text knows nothing about the retina display, so renders as usual for a regular one - doing antialiasing to smooth out the edges of text. The remote desktop/vnc app running on the local machine then takes that remote image and it must scale it up to fill the same physical area on the screen. So it doubles it and anti-aliases it again to smooth edges of shapes. It's this double anti-aliasing that makes text blurrier rather than smoother....
VNC viewed on a regular display:
VNC viewed on a retina display:
There is a general solution...
In Finder go to Applications, find the Remote/VNC program, right click and "Get Info".
Check on the "Open in Low Resolution", and things should be nice and crisp the next time it is launched.