Does high density altitude affect your landing speed?
Your $v_\mathrm{ref}$ does not depend on density altitude since it is given in Indicated Airspeed, which already accounts for density effects. However, the True Airspeed and therefore also Groundspeed will be higher at a higher density altitude, resulting in more runway required to stop.

The Flight Safety Foundation has a nice summary of factors influencing $v_\mathrm{ref}$:


  Factors Affecting the Final Approach Speed
  
  The following airspeed corrections usually are not cummulative;
  only the highest airspeed correction should be added to VREF
  (unless otherwise stated in the AOM/QRH):
  
  
  Airspeed correction for wind;
  Airspeed correction for ice accretion;
  Airspeed correction for autothrottle speed mode or autoland; or,
  Airspeed correction for forecast turbulence/wind shear conditions.
  
  
  Gross Weight
  
  Because VREF is derived from the stall speed, the VREF value
  depends directly on aircraft gross weight.
  
  The AOM/QRH usually provides VREF values as a function of
  gross weight in a table or graphical format for normal landings
  and for overweight landings.


(FSF ALAR Briefing Note 8.2 - The Final Approach Speed)

Altitude, Density Altitude or Pressure are not listed as factors affecting $v_\mathrm{ref}$.