If you look in the workspace, you should see that X has length 200 while V has length 101. If you want to plot V over X, they need to have the same length.
You create X by combining x1 and x2, both of which have length 100. Thus X has length 200.
You create V by combining Va and Vb, and while Va has length 100, Vb has length 1. Thus V has length 101.
Do you mean for Vb to equal P2 over the entire duration of x2? Then you could define it with this:
Vb = P2*ones(size(x2));
