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The answer lies in one of Magics most fundamental concepts, the stack
If you don't control both Fang and Vanille at that time, the ability won't resolve, you won't have the opportunity to pay , and none of the ability's other effects will happen
This is just the natural progression of number and land drops, but if you make a 3/3 Shark and they make a 4/4, 5/5, or even 6/6, its relatively easy to still trade Shark-for-Shark if you find a second Typhoon and double block
While the spell half of an MDFC might not be super efficient, drawing an MDFC for your turn is about a thousand times more satisfying and productive than drawing a random land is