“So vaci sankhara can be more specifically defined in terms of vitakka and vicara right? Then the key to stopping a certain thought is to stop the vicara, then vitakka, then apply vitakka to another thought.”
You got it exactly right!
P.S. For those who may not know: Vitakka is setting one’s attention on a given thought object. Vicara is maintaining one’s attention on that thought object, generating more vaci sankhara.
Vaci sankhara are actually defined as “vitakka, vicara“.