Jay Z -- Vegan in Vegas

Jay Z continued his 22-day quest to eat an entirely vegan diet yesterday in Vegas ... and then partied his face off while paying tribute to Beyonce's new surprise album.

Sources at Hakkasan restaurant tell us Jay dined on strictly vegan options after his concert ... while his friends -- including rapper/producer Timbaland -- ate crispy duck salad and roasted Chilean seabass.

Afterward Jay and his entourage hit up Hakkasan's nightclub (pictured above) ... and toasted champagne to honor his wife Beyonce's surprise album when the DJ played their new collab "Drunk in Love."

99 problems but a dish 'aint one.