If the cigarette butts are among small gravel, then a vacuum might not be the best idea, unless you want your robot's new nickname to become "ol' Clunky".
But if they are among anything larger (at least an inch or two across) rocks, and the vacuum isn't super powerful, then you should be fine. Vacuums do have the advantage of being able to suck up multiple ones at once, so they do present a nice solution. I'd do some testing first, though. Go to a local car wash* to one of the coin-op vacuum cleaners, and try sucking up cigarette butts from around the machine. (Alternately, you could try using the air intake on a leaf blower.) If it works well, then go for it and put it on your robot.
* An industrial strength vacuum cleaner or a leaf blower would probably be better handled to cope with rocks than your typical central vac or Dyson, so better to test it out with those.