Two congruent angles are always vertical angles?

Do you mean "Are two vertical angles always congruent?" Vertical angles are always congruent, but congruent angles do not have to be vertical. Any two angles with the same angle measurement are considered congruent by definition. The reason why vertical angles are always congruent is explained below.

Imagine (or draw) an X forming 2 pairs of vertical angles. ∠1 is to the left, ∠2 is on top, ∠3 is to the right, and ∠4 is on the bottom. Vertical angles are always congruent because ∠1 and ∠2 are supplementary, meaning that their measures add to 180 degrees. The measures of ∠2 and ∠3 also add to 180 degrees. This means that m∠1+m∠2=180 and m∠2+m∠3=180. Using the Transitive Property, it becomes m∠1+m∠2=m∠2+m∠3. If you subtract the measure of ∠2 from both sides, it becomes m∠1=m∠3. I hope that helped!