Do token units created from cards played from hand, for example Vanguard Captain, enter ready when Rek'Sai, Breacher is in play?

Yes — each Recruit Vanguard Captain makes is played, isn't played from a hand, so Rek'Sai's grant applies and each one can enter ready if you pay its Accelerate cost.

Two things to keep straight: tokens are genuinely played even though they aren't cards (Rules 350.2, 185.2.a), so the "played from anywhere other than a player's hand" condition is satisfied — the same logic that gives Hidden cards Accelerate (FAQ #12512). And Accelerate isn't free: it's an optional additional cost of [1] plus Power, and since Recruit tokens are domainless (Rule 187.1) that Power is [A] any domain — so [1][A] per token, paid during each token's pay-costs step (Rule 805.1.b.2).

Rek'Sai just has to be on the board when the tokens are played, and note it only covers units — spells and gear played off-hand get nothing.