Great idea. I do the same. There are times when I see blobs of fish waste hiding around plants and ornament rocks. Much cleaner way to do water changes and keeps substrate sand from crusting (I only have a school of pygmy corys that don't dig in the sand).I vacuum all my planted tanks, just don't do a deep vac around plants with "delicate" roots. To me rotting organics feed algae more than they feed plants... Just my opinion. Heck I even vacuum my MC, HC and glosso carpets... So much junk settles out underneath the "canopy" of the carpet.
With other plants I'll put the siphon over the substrate and swirl it around to kick up mulm and remove it.