Vallis are great, easy to care for plants. They don't need too much light, over 1 Watt per Gallon should be ok. They are different to most plants as they can obtain carbon from the water in the form of carbonates, for this reason they thrive in hard water. They won't do well in very soft water. They don't need a nutrient-rich substrate either as the leaves are responsible for gaining nutrients, that's why their root structure is fairly small, it's mainly for anchoring.
If successful Vallis will send out runners and propagate very easily, filling your tank if not kept in check. I throw out about 5 new plants every week!!
Summary - Enough light and hardish water and they should do well.
edit -Twisted Vallis (Vallisneria torta) prefers harder water still to Straight Vallis (Vallisneria spiralis)
Good luck