I've heard that the strings in String theory are vacuums, is that right? And if so then how is it expected they form?
The strings in string theory are rolled up M2-branes (i.e. two dimensional branes). M2 and M5-branes are supersymmetric p-brane solutions of D=11 supergravity, the low energy approximation to M-theory. To fully explain the origin of these p-brane solutions requires a complete version of M-theory.