how to plot a vector function

Answer 1

Plotting of a vector function is e.g. here:

http://fenicsproject.org/documentation/dolfin/1.4.0/python/demo/documented/stokes-iterative/python/documentation.html

In this case, you can use just a very simple plot(u) command.
To see more possibilities, go to the documentation page of plot command.
You will see there more options of this command (at the bottom of the page).