I would suggest there are a couple things going on here. First on the light, yes the duration was excessive so that may help. The type of light is also an issue, as I detailed previously (post #14). A high blue will encourage algae because plants cannot use all of it (if red is weaker) but algae can. I offered a couple options.
Also on light, bright daylight in a room can encourage algae. For three summers I experienced an increase in BBA in my tanks, until I finally clued in to the fact that the longer and stronger daylight during the summer months was just enough to upset the balance. I installed heavy window coverings, basically a blackout in fact, which I was able to do as the tanks were in a dedicated fish room, and that was the end of the summer algae bloom in successive years. But this shows how easy it is for algae to take advantage.
As for fertilizers, the substrate tabs are better when it comes to algae issues because they do not break down into the water column. At least the Seachem Flourish Tabs do not; I don't know how they achieve this, but in my experience it is true. A couple of these might improve your Vallisneria.
As for using both tabs and liquid, that depends upon the specifics of the aquarium. Obviously floating plants and plants not rooted in the substrate can make no use of substrate tabs. But there is also the fact that nutrients occur naturally, from the fish (food = waste = nutrients), and water changes (replenishing the "hard" minerals like calcium and magnesium). The plant species (slow-growing use less nutrients than fast growing) and the numbers also factor in. In some tanks just the tabs may be all that is needed; in others like mine I need both. There is no "one size fits all" here, beyond the fact that it is the light/nutrient balance that matters and once that is established for the specific situation in an aquarium, you will never see problem algae.
