Get some smaller tubing, perhaps as small as air tubing, or maybe the next size up. It is available in hardware stores. You could simply remove the intake portion of your siphon and use that tubing.
Keep a finger over the tubing. Keep your other hand wrapped around the outlet end of the tubing, ready to pinch it off.
Get a siphon started (Fill the tube with water, but keep both ends closed) and put one hand and the tubing as deep into the plants as you can without getting too close to the substrate.
Remove your finger from both ends, this starts the siphon, and move around through the plants.
If a problem happens (and it will!) pinch off the outlet end of the tubing to slow or stop the siphon and clear the problem. Often it will be a granule of substrate, or a leaf getting sucked into the tube.
You will get good at keeping the finger ready in the tank to shut off the siphon, too, when you see a problem getting close. In my case it is usually curious fish.
You cannot use this method to vacuum into the substrate, but you can get a lot of the debris that is sitting on top of the substrate.
To vacuum the substrate you could try either of 2 things.
They sell smaller siphons, or you could put some strong mesh over the tubing. Does not work very well. Mostly there are so many plant roots you don't really want to deep vacuum the substrate anyway. You can get to the open areas where there are few if any roots.
How to revamp the tank:
If you are up to redoing the whole thing:
Set up several shallow dishes. Mixing bowls or something. Fill them with water from the tank, a couple of inches deep.
Pull out the plants and sort them by species, and perhaps size.
Take this opportunity to deep vacuum the substrate, or get new if you want. Pretty much empty the tank of water. You can add more water and vacuum the substrate again and again, if needed.
Put some slow release fertilizer deep in the substrate.
Then replant: First the driftwood, then the plants.
Group the plants so the same species are together, not mixed. If you have 2 main species then start them back left and back right.
A third (lower growing) species can be in the front.
Careful not to disturb the fertilizer tablets. You could add these last, if you want, pushing them all the way to the bottom.
Keep most of the area clear. You can heavily plant the back half of the tank with the best of the plants you have saved. But leave the front half open, with just the lowest plants. I am not saying draw a straight line, just a general concept. Probably bring the plants that you are using in the back forward along the sides some. Like you are making a large arc.
Mist the plants often as you work.
Refill the tank by placing a plate or plastic bag over the substrate and pouring the water in slowly. This will minimize clouding.