Okay... 1, Vallis don't need high light. Medium light is fine.
If you have low light, you could try Echinodorus Augustifolia var. Vesuvius
(Everywhere I read says it needs high light, but from my experiance all of the echinodorus can do well in low light.)
Of course, low light is about .75 to 1 watt per gallon...
2. I love stem plants, but hornwort and anarchis are the worst. I just tie them to a rock. I've nver had the stem rot where it's tied though... Maybe you tied it too tight and crushed it?