I hope this is not homework. If it is, you cannot ethically use this solution.
Break it up into its component functions, then use logical indexing for the threshold condition:
f = 1E+3;
vr = 0.72;
tau = 1;
vi = @(t) sin(2*pi*f*t);
vd = @(t,vr) (1-vi(t)).*(vi(t)>vr);
vo = @(t) vd(t,vr).*exp(-t/tau);
t = linspace(0, 2, 150)*1E-3;
figure
plot(t, vi(t), t, vo(t))
grid
That ran without error. Check it to be certain it matches the functions.
