No, I understand what you are saying. The fuel sloshing has nothing to do with whether or not there is pressure or vacuum in the tank. If what you are saying was true, even the later setup would blow fuel out with fuel slosh. I still say you have the wrong cap or it's junk, fuel sloshing shouldn't open the vent on the cap, regardless if the tank is vented or not and if the tank is vented with a hose all the more reason it should never open the cap vent.
I would put a mighty-vac on the cap and see how much vacuum it takes to open it.
Personally if it was me, I would plug off that vent or run it back to the top of the tank, having a hose routed like you described is not a very good vent and if fuel gets into it, it won't vent anyways, along with the fact it must reek fuel fumes all the time.

Mark D.

1968 W200 Sweptline
1969 D100 Utiline
1971 D100 SE Sweptline
1993 D250 Club Cab Sweptline
1986 W250 Cab & Chassis
1978 D300 Sweptline

E. Brookfield, Massachusetts