Dipole in a Uniform Electric Field

In a uniform field, the charge $+q$ experiences force $qE$ along the field and $-q$ experiences force $qE$ opposite to the field. The net force is zero.
(In general $F = p\dfrac{dE}{dr}$, which is zero because E is constant.)
Potential energy of a dipole: $U = -\vec{p}\cdot\vec{E} = -pE\cos\theta$
When $\vec{p}$ is along $\vec{E}$, $\theta = 0^\circ$ and $U = -pE$, which is the minimum possible value (stable equilibrium).
So the force is zero and the potential energy is minimum.

The two charges of the dipole experience equal and opposite forces qE, which form a couple.
Torque magnitude $= qE \times 2a\sin\theta = pE\sin\theta$, directed perpendicular to both $\vec{p}$ and $\vec{E}$.
In vector form: $\vec{\tau} = \vec{p} \times \vec{E}$
(The dot product $-\vec{p}\cdot\vec{E}$ gives the potential energy, which is a scalar.)