My first try at vallisneria failed and I learned the hard way that it's really important not to bury the bulb, only the roots. "But my new plant has no roots attached, what do I do?!" you may ask. In that case, let it float in your aquarium for a few weeks until roots appear. Once you have those, you can bury them in the substrate with the bulb on top.
* also, in my experience, vallisneria will do best with nutrients through its roots as well as in the water column, so get some tabs to add.