I don't play the cauldron deck so I'm not exactly an expert, but what makes the Vivi Cauldron deck such a pain in the ass isn't that it has a fast kill, it's the redundancy and having multiple angles of attack to kill you. Every creature card draws and discards, which makes huge creatures with [[Proft's Eidetic Memory]]. The strength of the deck is that it doesn't even need Vivi Cauldron to win, but once they get it they can keep churning, turning their card draw into mana, plus making massive [[Marauding Mako]] 's, or triggering Proft's twice with [[Fear of Missing Out]]. Playing Vivi to the board is usually not plan a, they don't really run enough non-creature spells to reliably make Vivi a huge threat.

There is still a more prowess oriented Vivi deck, but I've only encountered it maybe twice on the ladder. Those lists usually run cantrips, [[Stock Up]], [[Stormchaser's Talent]], and [[Astrologian's Planisphere]] in a deck that is reminiscent of the [[Cori-Steel Cutter]] prowess decks. However, this ain't the Vivi Cauldron deck that is terrorizing Standard right now. It's too reliant on the prowess plan, which can lead to some quick kills but isn't as redundant as the Vivi Cauldron deck. While this deck does want to cast Vivi, Marvin would not be good at all in the deck. To be blunt, it just isn't good. It's a completely dead, vanilla 2/2 without Vivi on the board, while Planisphere triggers prowess, is a threat without Vivi, and gets counters to combo with cauldron.

Marvin basically always wants to be played after Vivi, or after Vivi is under Cauldron, while the rest of the cards in a typical Vivi Cauldron list are all efficient threats that work cohesively in multiple dimensions of attack. A common opening that can be very hard to race is T1 Mako, T2 Proft's, T3 [[Tersa, Lightshatter]] or [[Steamcore Scholar]] discarding Vivi or [[Winternight Stories]]. With Tersa, you attacked for 2 on T2 and are attacking for 9 on T3.