Regardless of whether Vallisneria looks sad or glad when you get it going to happen is you'll plant them and roots will grow while it recycles the leaves as it literally pulls the chemicals from out of them from the end of the leaf on down towards the crown, one by one when it's done you think you have nothing. It's not a ransom decay but a precisely controlled programmed death s that nutritional component such as the very precious (to a plant) nitrogen can be used and about this time you'll at that point notice tiny plants, a few of them. In a couple of months you'll be cursing the prick you got this cursed weed from.
There's no roots, that's ok, float them till at least three roots are two inches long, then ou can plant them. They're a bit hard to keep in the sand until they've rooted, but you can use a couple of small pebbles to hold them in place till they do.
You don't really need to fertilize them until after the old leaves die and new pants start forming.
What is helpful though it sunlight. These plants live in deep to quite deep water (for plants) and they do best in the full sun where they're found. You can't have enough light for these and they're what I put in parts of any tank exposed to sunlight.