I suspect that the terminal on the rear of the amp labelled "gnd/earth" provides a circuit ground point (as opposed to a chassis ground point), to which the sub's negative input (i.e., its circuit ground) can be safely connected. If you can't readily get confirmation of that from Vitus, you can confirm it by using a multimeter to check for continuity (zero ohms, or a tiny fraction of an ohm) between that point and the ground sleeve of an RCA connector on the amp. (Have nothing else connected to the amp if and when you make that measurement; otherwise the results might be misleading). If so, you are good to go; for each channel just connect the corresponding sub's negative input (the black wire) to that gnd/earth terminal, and its positive inputs (both the red and yellow wires) to the amp's positive output terminal for that channel.
If the multimeter indicates a low resistance (say 10 or 20 ohms or so) rather than 0 ohms, that connection arrangement should also work fine.
If the multimeter indicates a very high resistance, or an infinite resistance, then I would suggest that you obtain an RCA plug, solder the black wire from the sub to its ground sleeve connection point (while leaving the center pin unconnected), and insert that plug into an unused RCA connector on the amp. That will establish a suitable circuit ground connection.
Disclaimer: I have no experience with either Vitus products or REL subs. My comments are based on electrical engineering principles, which I feel certain are highly applicable to this particular situation.