Which Vectric package are you using?
Most of the Vectric software has 'Node editing' available by selecting the vector and a right mouse click. In the drop down menu select 'Node Edit' and when the nodes are visible on the circle (or other vector you are working with) by clicking the node you can select it and then right click for an option from the next drop down menu. Choose 'Cut Vector', and then do the same with the node directly opposite that one(180º from it) and now you have 2 half circle vectors to work with(mirror half etc). I hope this helps.