Air carrier Captain reported ground crew failed to maintain a safe zone in poorly marked; dimly lit gate; resulted in stopping aircraft.

Narrative

While parking at the Gate Lead Marshaller was directing forward and cleared into gate. Both left and right wing walkers had their wands up signaling it was safe to continue. However; a vehicle from the left and right passed both wing walkers and failed to yield. They failed to stop the vehicle and the lead marshaller continued to signal the aircraft forward. I stopped the aircraft and flashed the landing lights in an attempt to signal the vehicles. Both vehicles failed to yield and proceeded in front of me through the assumed safety zone. Operations was notified. Vehicle traffic failing to yield to aircraft and ramp crew failing to establish a safe zone for aircraft movement. These gates are hard stands at the airport. They are poorly light and markings are not visible at night in raining wet conditions. Better markings and red safety zone lines would help. Ramp crew need to be assertive with ground traffic and stop traffic when aircraft are approaching to park. If a safety hazard presents itself the ramp crew need to signal the aircraft it is no longer safe to approach and signal to stop the aircraft until they have established safety for the aircraft.