JVP is almost exclusively a concession to Shops and Thorn decks. It helps you hit your land drops in the early game and flashback removal spells in the mid-to-late game. However, the way you have constructed your deck, your primary removal spell against Shops is Grudge and you don't gain as much from JVPs flashback.
Snapcaster Mage is better against Combo and Gush decks. The 2/1 body is not really aggressive, but readily sacrificed to flashback Therapy and you gain the ability to Snapback Mental Misstep and Flusterstorm. It depends on your metagame but the way you have your deck built right now, I would keep the Mages
Sultai Charm was talked about quite a bit when it was spoiled but it seems most people reached the conclusion that it's too expensive to serve as Shops hate and not impactful enough in other matchups to justify it's inclusion. You want Sulfur Elemental over both Virulent Plague and Illness in the Ranks. The deck is called Grixis Pyromancer for a reason and you really don't want to shut down your own Pyromancers.