hard to say without seeing what you're calling too big xaml lines.
But to answer the second part of the question : in your project, just add a new UserControl. Just copy/past the Design stuff (take care about copy options in Expression Design !) into the new control.
Then, i your main canvas, just drag and drop an instance of your new user control... The xaml of the main page will stay very short.
Just another idea : vector is cool of course, but in many circumstances it is more efficient to export to PNG from Design and use the latter. A PNG can be very little, with transparency, and need no CPU on the host. It can even be resized (a little) without
any visible effect. I personnly often use PNG instead of vector when I have complex drawing in Expressiono Design (and also when I'm using features that do not exist in Blend, such as brushes, a lot of effects,etc).