I don't have time to go into depth, but I think the main thing you're looking for is that the water pump type pump, with an impellor, is really only good for creating flow. It's a non-positive displacment pump and generally speaking, only good for ~100 PSI. The fluid is drawn in the inlet, and pushed around and out the outlet. But if there is too much resistence at the outlet, say, too much pressure in the system already, the fluid won't go out the outlet, and will just stay circulating in the pump.

The vane pump is a positive displacement pump. turn the shaft while you have it apart and you'll see the "chamber" (the space between the vanes) grow and shrink as the rotor turns. It grows as it passes the inlet, allowing fluid to be drawn in, and then shrinks when passing the outlet FORCING the fluid out. This type of pump can generate much higher pressures, usually several thousand PSI. If perchance the fluid in the system (at the outlet) is already too high a pressure already, there is not option for the fluid to stay in the pump, either the pump stalls (doesn't turn any more) or something gives (think big oily mess). That's why these systems have release valves.

MV