Because viibryd (vilazodone hydrochloride) acts in 2 ways on serotonin..ssri plus buspar why is it so stimulating?

Buspar (buspirone) is an agonist: Buspar (buspirone) is known as an agonist on 5-HT1A and therefore a direct anxiety anxiety relief. There are 11 HT receptors, and only this one directly affected by Buspar (buspirone).

Brains are sensitive: You are correct, the drug Viibyrd (vilazodone) is a serotonin reuptake inhibitor (like Prozac/Lexapro) which results in more serotonin available for the brain to use. Buspar (buspirone) is usually used for anxiety. In some, these two serotonin effects are too much/overstimulating. This effect would be highly individual and would not occur in all taking both drugs. Good wishes:)