I didnt test vd but 100 monsters soloed in 7 mins is really fast, for new players with G4 set will take much longer even with tank, 15 mins probably?
My idea behind this is to bring player back to open world lving instead of dungeons, make them reward more from world mini boss since their exp is a joke compare to VD.
Base on Keiths info, I prefer 500 VD monsters for a map. This would probably take like one and half hour of training for most players(1 dps).
Those maps are extra things when we level up, dont think its a good idea to make it too easy to drop, but I do agree to increase their difficulty and value, like better chance of getting rare rings&boxes. Monsters between lv100 to lv130 should have a chances drop medium map besides low map. lv130+ monsters should drop medium map and have a low chance to drop high map.