Legendary Creature - Minion 1BR
As Xantcha, Sleeper Agent enters the battlefield, an opponent of your choice gains control of it.
Xantcha attacks each combat if able and can't attack its owner or planeswalkers its owner controls.
3: Xantcha's controller loses 2 life and you draw a card. Any player may activate this ability.
Yes, this is a Commander-targeted card, but hear me out.
One of the biggest challenges with building a direct-damage-based competitive Vintage deck is the lack of viable card advantage engines. Against control decks you'll burn through your hand, only to have the opponent stabilize and run away with the game.
Xantcha plays double duty: it lets you funnel surplus mana into refilling your hand, and also, once it's in play, gives you a repeating source of direct damage. Yes, your opponent can also activate Xantcha's ability, but how many cards are they going to draw when they're at 10 life and you're threatening to burn them out?
There is another synergy worth mentioning:
Bloodchief Ascension has been on my "unbroken" list for years: cards whose effects read as extremely potent, but never made a splash in Vintage. Ascension unfortunately suffers from the rampant Missteps in the format, but once online it is an extreme threat: it's a Kembal that also punishes cracking fetchlands, getting Wastelanded, dredging, discarding to hand size, etc etc.