The vasa vasorum, the collection of small arteries and veins supplying blood to the smooth muscle and fibroblasts within blood vessels, is found in the

The vasa vasorum, the collection of small arteries and veins supplying blood to the smooth muscle and fibroblasts within blood vessels, is found in the tunica externa.

Explanation:

In order to understand the answer above let explain the concept of tunica externa

This in Latin means outer coat and can also be called 'tunica adventitia' which also means "additional coat in Lain",it is the outermost layer of the a blood vessel covering the tunica media(This is a Latin word  translated as middle coat in English which is the middle layer of an artery or a vein it lies between the tunica externa  and the tunica initima which is the innermost part of the artery or vein).

An Artery

We can define an artery as a blood vessel that transports blood from the heart to other parts of the body.

An Vein

We can define a vein as a blood vessel that transports blood to the heart

and we can define  blood vessel as a tube that carries blood in the circulatory system.  

thermal energy is the answer