I don't think there could be a liquid return line to store the excess, since petrol tank when open is effectively at atmospheric pressure and doesn't have enough energy to be "pumped" back up, right? ... Only one possibility is to have a suction pump in the dispenser, which obviously isn't the case. If that was the case, petrol pump can suck out petrol whenever they want, regardless of the fact if the tank is full or not.
It can also interpreted that petrol is present in the vertical section of dispenser after it has been metered, say quarter of a litre, and when fuel is cut this is returned back to the tank to avoid fumes coming out of the nozzle. If that's the case this amount will always be constant, even if you fill half or full, ... In fact if the tank is filled multiple times, the loss would add up, the only way to minimize it is to empty the tank and full to the brim next time. However I think fuel regular fuel cut-off is at the nozzle and not the dispenser top. There may be additional cut-offs for safety though, not for operation.
There is vapour return line when tankers are being filled at the refinery, however there is no return line in petrol dispensers we see at regular petrol pumps, The fumes go out in the atmosphere ... There is a second sensing line inside the nozzle, which is closed at the cut-off assembly. When the tank is full and petrol starts to rise up this tube, the pressure increases slightly and shuts the fuel. That can be seen in action when you ask to fill the tank and dispenser automatically stops.
There is another way of looking at it, vapours are lost, whether to fill it on low or high, high flow would be more turbulent and produce more vapours, but since its faster, the process would be of shorter time. On the contrary at low flow rate, vapours production may be low, but that would be for a longer time. So what's better can only be calculated by getting all the variables for a particular situation.
In all the cases though, what I feel is, the loss might not be significant. We'll be paying more price in the form of lost peace of mind than lost fuel vapours 
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