Richard Sherman -- Plays Second Fiddle ... To Insane Street Violinist

Seattle Seahawks superstar Richard Sherman was stopped dead in his tracks in Vegas this week ... all thanks to a crazy talented street violinist in Vegas.

Sherman and ex-teammate Brandon Browner were strollin' down the Vegas strip when they came across Tony B ... a 23-year-old pro violinist who was jammin' out in the middle of the day to make some extra cash.

The NFL stars were so impressed, they stopped and hung out for a while ... with Sherman even recording some of the sidewalk show to share on Instagram.

Vegas baby.