I'm showing in the greenhouse that vals with really low nutrient availability and direct sunlight have very low growth. My Vallisneria americana should be getting leaves about 4 ft long but they are 2" at most after 2 years of growing. Unless you have sunlight, it is probably a nutrient issue.
So your pH is around mine, off tap I run 8.0pH already, and I don't run any nutrients in my tank albeit I have a different vallisneria (nana), but grew probably 60 nodes in a mixture of dirt/fluval stratum/eco-complete/botanicals in a span of 4 months. I want to say your lighting may not be that great if it's not growing tall. I have the opposite in my 55 where I do grow jungle val, and they're growing super tall, but they're not wanting to propagate. I also have leopard vals that are doing super well, so I guess it all depends.
They need extra iron, light and root tabs. I grew them in my refugium and threw out a garbage bag every 3 weeks. Super fast growers. the Jungle Val was growing 3+ft for me. Quit growing them because they hate Excel and I was having issues with staghorn algae at the time. I had lots of fish in the main tank providing nitrogen so you might need to add flourish or something on top of the root tabs. I run my tank at pH 6.6 to 7 with CO2 so might not be comparable to yours. Wiki says they grow to pH 8.5 but I think 8 is high for a planted tank. Not sure this is any help but might give you clues. I had your results before adding root tabs and Iron.
I apologize if this seems like a dumb question, but are you sure that what you have is actually Vallisneria? Maybe you've got Helanthium tenellum (formerly known as Echinodorus tenellus), or Sagittaria subulata (dwarf sag.)?