Once you've landed on an adventure card that falls in the discover range, you can either cast it as normal, or cast the adventure spell if its mana value also falls in the appropriate range
Vindictive Vampire 4 mana is a lot, but in sacrifice decks, sometimes you really, really need one more card that drains and gains, and Vindictive Vampire is happy to be that card if you need it
If you were calculating the odds of opening a given card, this would be 50
In addition to the 275 brand new cards in the main set, there are 63 Mystical Archive cards (reprints of iconic sorceries and instants with special frame and alternate-art) that players can get from Strixhaven Draft, Set, and Collector booster boxes
Definitely not, even if I supplement the strategy with a bunch of older Food cards
Well, you pay 5 mana for a net gain of only one cardfor a whopping 5-mana-per-card ratio