I would say Vallis didn`t thrive in my tank, but that is relative. It is a 3.4WPG EI tank and the Vallis had everything it could possibly want, and it was also the first plant I ever saw pearl. Nevertheless, compared to my other plants it didn`t grow as quickly.
Without the light and ferts, you may struggle with soft water, but why not give it a try. No two tanks are the same.
I left on or two of the healthier runners in there for a while and they started to thrive, so maybe after a little struggle at first you could end up with some healthy Vallis, as per Darwin.
Give it a go, I say.
Dave.