I use a MK3 heated bed for my Vertex printer, this is how I changed the configuration in Reptier-Host for it.
The first step after installing the heated bed, is to change the G-Code used by Reptier to enable the heated bed. This is done by pressing the "Slicer" tab , then "Configuration" in Cura settings under "Print" you then select "G-Codes".
In the "Start G-Code" you need to find the line with "M82 ; Extruder in absolute mode" after that you append the following lines
M82 ; Extruder in absolute mode
; Enable heated bed
; Activate all used extruder
In the "End G-Code" you need to ensure that there is a line that turns the Heated bed off again:
G92 E0 ; Reset extruder position
M140 S0 ; Disable heated bed
M84 ; Turn steppers off
Mine had this as the default.