If you have some driftwood or rocks in the tank, you could always attach some java fern or anubias to them, they just cling to rocks and stuff (I don't know what the term is for the aquatic version of an epiphyte...), so they don't really have any substrate requirements.
it's a bit more effort, but you could also try keeping a bundle of substrate near the roots. I can't remember specifically what I used, but it involved filling one of those little pressed-peat pots with some mix of sand, vermiculite, and clay around the root-ball of the plants. Then just bury that bundle in the gravel.
In a low-tech tank, that should (hopefully) provide enough nutrients until the mulm/detritus builds up in the gravel.
Oh, and stop vacuuming the gravel, if you are doing that now.