I have heavy clay soil, and I need to keep it covered with at least a shovel depth of layers of mowed weeds, well-composted manure and straw all year long. If the sun gets to it, it will seize up, or if the rain pounds it, it gets super hard. My thick, layered raised beds (no wooden sides) have been a godsend as far as saving water, absorbing fog and dew, keeping weeds down, and even faking out some mice and voles that root around underneath it.
The thing about green manures is they had to use some of the nitrogen/nutrients in that soil, and are uncomposted greens, so until they break down (and it takes extra nitrogen to break them down over possibly 6 months or more in passive raised beds) they will actually use some of the nutrients you think your vegetables are getting until they are broken down, and THEN your veggies get the added value. They are a longer process than we are led to believe. Just tilling them in, turning them into browns, and assuming they are letting off nitrogen/potassium/phosphorus and micronutrients as soon as they are under the soil?? that is never the case, right? Even with compost. The browns can absorb water that has nitrogen dissolved in it (in the form of urea), but they aren't going to let go of it until they rot away.
So they should either be broken down in the off season, or piled in a raised bed so they will eventually break down, or composted elsewhere with other things and moved when finished composting. Greens can heat up a compost pile very nicely, and the browns in the compost can absorb that nitrogen gas as it is released from the cells breaking down in the greens, instead of going strraight up in the air. Even if you are doing nitrogen-fixers, those pin-head sized nodules on their roots can take 6 months to break down under the best of conditions, assuming you mowed them and left the nodules under ground, plus the greens still need to break down. I absolutely rely on green manures, but they are a down-the-road method.
compost teas, thistle teas, stinging nettle teas, manure teas, those will help right away drenched into the soil, and then covered with a good depth of mulch that is maintained all season.
I get whole fields of knee-high vetches, but they aren't enough by themselves to provide everything to a hungry vegetable. I still make layers of greens and browns in my raised beds, but I always provide extra compost and composted manure, compost teas during the growing season.
"Life flows on within you and without you"...George Harrison
Coastal California, USA, Mediterranean climate - no summer rain, a little frost mid-winter