I haven't ever tried it in the Vedder, but I did it lots this summer for summer run steelhead.
Most of the flows I nymph fished on were smaller flows, where the steelhead were quite picky after being in the system for a while.
It would be a lot more difficult in a flow the size of the Vedder and with the water clarity it has.
It would be much more effective and efficient to swing intruders (cover water fast and make sure the fish can see it).
If you were to nymph fish, I would go to the upper river in low clear water periods late in the season (March/April).
Cast upstream and allow it to bounce bottom. What I have found with trout and steelhead alike you need to be RIGHT on bottom for a good success rate.
I had VERY good success on this pattern (my variation of the chain gang stonefly):
Other things I would try would be caddis nymph patterns and glo bugs.
Cheers, and good luck,
Dan