Most likely you have done this by now, but a bought a wet/dry vac. I got tired of scratching my glass when trying to get it out, and it is a lot faster to complete the job.
Yep, I took out the plastic vacuum part from my gravel vacuum, started a suction with just the pipe... held it below the sand....sand got sucked right in PERFECT...and FAST... every time the bucket was a bit full of water, I let the water rest...sand goes to the bottom, empty the water back into the aquarium...
took me around 1 hour since my vacuum is not meant for a 55gal, more of a smaller tank vacuum... but got the job done