Question

What is the difference between protonephridia and metanephridia??

Answer 1

Protonephridia are made up of many flame cells. In the flame cells, there are cilia at the 'tips'. They beat, creating a sort of vacuum that sucks out substances on the other side of the membrane. This gets filtered, things are reabsorbed as appropriate & the waste is excreted through the nephridiopore (your one opening). This is mostly for osmoregulation.

Metanephridia is in segments, there's an internal opening (nephrostome) & an external opening (nephridiopore). Body fluid is drawn in from the nephrostome, substances are filtered, selected or reabsorbed as necessary. The wastes are excreted from the nephridiopore. This functions in both
osmoregulation & excretion. And yes, as you said, this is vascularised for secretion/reabsorption purposes.