  We consider the two-dimensional incompressible Euler equation \[\begin{cases}
\partial_t \omega + u\cdot \nabla \omega=0 \\ \omega(0,x)=\omega_0(x).
\end{cases}\] We are interested in the cases when the initial vorticity has the
form $\omega_0=\omega_{0,\epsilon}+\omega_{0p,\epsilon}$, where
$\omega_{0,\epsilon}$ is concentrated near $M$ disjoint points $p_m^0$ and
$\omega_{0p,\epsilon}$ is a small perturbation term. First, we prove that for
such initial vorticities, the solution $\omega(x,t)$ admits a decomposition
$\omega(x,t)=\omega_{\epsilon}(x,t)+\omega_{p,\epsilon}(x,t)$, where
$\omega_{\epsilon}(x,t)$ remains concentrated near $M$ points $p_m(t)$ and
$\omega_{p,\epsilon}(x,t)$ remains small for $t \in [0,T]$. Second, we give a
quantitative description when the initial vorticity has the form
$\omega_0(x)=\sum_{m=1}^M
\frac{\gamma_m}{\epsilon^2}\eta(\frac{x-p_m^0}{\epsilon})$, where we do not
assume $\eta$ to have compact support. Finally, we prove that if $p_m(t)$
remains separated for all $t\in[0,+\infty)$, then $\omega(x,t)$ remains
concentrated near $M$ points at least for $t \le c_0 |\log A_{\epsilon}|$,
where $A_{\epsilon}$ is small and converges to $0$ as $\epsilon \to 0$.
