What it does
"Vanguard Species" isn't an official Magic keyword or ability. It may be a card name, a flavor term, or a misremembered mechanic—but it doesn't correspond to any rules text in the game.
If you're thinking of a specific card, you might be recalling something like a creature with a tribal or "lord" effect (boosting creatures of a shared type), or perhaps confusing it with mechanics like "Champion" or terms from a particular set's flavor. There's also the old "Vanguard" casual format, which used special oversized cards that altered starting life totals and hand sizes, but that's a format, not a keyword.
If you can share the exact card or where you saw the term, I can give you the accurate rules and Commander context. Double-checking the precise wording on Gatherer or Scryfall is the safest bet before building around it.
Iconic cards with Vanguard Species
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