Hello Chopstick. The best I can tell you is "very carefully".

When I vacuum my 155g (note I don't have a planted tank, but plastic plants) I just accept the fact that the bottom is going to be a mess and have to rearrange things.
Take your time. Vacuum 1/3 of the tank at a time and don't try to do it all at the same time. I do a
water change and vacuum every 10 days so by the end of the month the entire tank has been done. Wish I could help you more, but I'm sure you'll get some more responses. Try to keep it fun.
Ken