We had good results in 2014 (
https://youtu.be/tPNvIXTAd4s), but decided not to use vacuum because adapting a FIRST-legal motor to a small shop vac was beyond our resources at the time. But another team won our regional with a vacuum pickup. That was Aerial Assist & the game piece was a giant yoga ball with a fabric covering.
I recommend high flow/low vacuum like a small shop vac produces, rather than low flow/high vacuum, as you would get from a venturi-type vacuum generator. Those things are small and light but require a LOT of pressurized air to work.
To prototype just cut a hole in the bottom of a plastic pail or bowl & duct-tape a vacuum hose to it. Even a cheap vacuum can generate ~60" of H20 vacuum (about 2psi). With a 6" diameter bowl: 2 psi x 3 x 3 x pi = 56 lbs lifting force! Even at 50% loss due to leakage, etc. that's still 28 lbs. You might need to vent your system to keep from collapsing the boulder.
Happy prototyping!