Some people do trim Vallisneria by cutting the leaves to the desired length. The plant will keep on growing longer leaves, and the trimmed leaves do not look as nice as untouched ones. It may be better to get a variety of Vallisneria that does not have such long leaves. Jungle Val can get leaves over five feet long!. As far as propagating, give the plants decent light, and they will send out runners. Usually, the problem with Vallisneria is that it propagates too rapidly and you have to keep pulling plants out of places where you didn't want them to spread.