I stir it up first, then give it a few mins to settle down. Than take the hose off the gravel vac, and just syphon off all the cr@p (which will settle on top of the sand) into a bucket (doesnt matter if sand gets sucked up. Then i rinse out the sand under the tap and put it back.
ive got a battery vac whay i do is try to get the vac just far enough away to seperate the sand from the crud then swoop in and get a bit of sand with it. tho i have thaught about making a gravel bag that lets sand thru.