The Marquee tool is for bitmap objects only; it won't work on vector objects. To remove part of a vector, you could use the knife tool to slice across the path in two locations, then delete the sliced section, or draw anoter shape, place it over top your original vector, select both objects and choose Modify> Combine Paths > Punch. In your example above, either of these options would work, but the Punch option would give you a complete closed path.
Thanks for the reply, although i have seen this done, here https://www.youtube.com/watch?v=5l0ghrx39M8 and have managed it once with no idea how i did it.
Punching or cutting doesn't give the same results with the sharpe flat edge one side and the gradient blur the other. If you see the vid on youtube via the link above, you will see what i mean.
He applied a bitmap filter to the ellipse, whcih automatically converts the ellipse to a bitmap. Watch the layers panel as he creates the oval, then applies the blur. You will see the object icon change in the Layers panel. Same thing happens when he duplicates and applies filters to the duplicate rectangle.
Aha....I had been using the vector properties panel to apply the blur, but for some reason going to the Filters menu and applying the blur from there causes the conversion to a bitmap, as you said.
I find that a bit odd!
Thanks for taking the time to answer Jim, you have been a big help.
The Filters menu has always applied bitmap, rasterized (destructive) effects. Live Filters in the Properties panel gives you always editable, all the time, filter effects, including the ability to stack and reorganize any effects applied to a vector or bitmap object. While many of the filters seem the same, you'll notcie there are some filters available in Live filters that you don't find in the main Filters menu.
The Smart Punch and Smart Knife extensions can help you make these kinds of edits:
You can't use a bitmap marquee on top of the ellipse to punch it (since the ellipse gets deselected as soon as you switch to the marquee tool), but you can just create a rectangle over one half of the ellipse. Then select both and run Smart Punch to remove that half.
Smart Knife works similarly, but with an arbitrarily complex path, and leaves two (or more) pieces behind after the cut, instead of deleting pieces.