Hi Bass45,
could you let me know what rig you are using for fishing the vaal? I don't mean what rod, reel and line, but rather do you fish with a strike indicator and some nymphs under it, or do you fish one of the short line techniques, ie just nymphs and no strike indicator? The Short line techniques are known as Czech Nymphing or similar things.
These are the two most commonly used techniques for fishing for smallmouth yellowfish and both have advantages.
My own personal preference is to not fish without a strike indicator and use a heavy fly as one of your flies. This is known as the control fly as you use it to control the drift of all three of your flies. Usually casts with this style are very short, eg a rod length or even less (but long casts can be effective when you get the hang of it). Smallmouth yellowfish are not always easily spooked so you can get quite close to them without them swimming off. I have often turned around to see a fish sitting in the eddy that i was creating with my legs.
The short line technique is simple. The flies are "plopped" upstream and then you follow them downstream with your rod, always keeping in contact with your flies. You will be able to detect takes a lot easier in this manner as you will see the line straighten or even feel the takes. Then you just lift the rod and you will be in on the fish. A big advantage of this style is that you can easily adjust the depth that your flies are fished because there is no strike indicator that will stop them from reaching the bottom (which is where the yellows usually feed). Once you get confident with this you can start to impart movement to the flies while they drift, eg jigging them up and down a tiny bit, or shaking the rod slightly while they drift etc.
I have not used the strike indicator technique much as i feel that you are not nearly in as much control and for a beginner you will miss a lot of the subtle takes where the indicator hardly moves. If you are using this method you may also be missing fish by fishing too far away from yourself. The more line you have between you and the fish, the more things there are to go wrong, like loss of tension and currents pulling your line in unnatural ways (we call this drag) which causes the flies to move unnaturally.
The advantage is that you do not need to have constant contact with the flies and you can cast to fish that are further away. If fish are not feeding on the bottom (which is not the norm but happens) then a strike indicator can keep your flies at the correct depth a lot easier than when short lining.
Yellowfish have quite soft mouths so it is better not to strike too hard. You only really have to lift into them and you should be vas enough. If the fish runs off let him go - if you try stop him he will come off. Once he has slowed down a bit you can apply a bit more pressure to get him back to you.
When selecting flies, i would not be without a hotspot nymph like the hotspot pheasant tail nymph or any other brownish coulored nymph with a bit of orange as a hot spot. Yellowfish seem to love a bit of orange, and recently i noticed that pink is a good colour too, but only in small amounts on the fly with the rest being natural colours. Buy some heavy flies too that will you can use as your control fly to get them down quickly. A good fly to use for this is the Mustard Caddis.
Hope this helps - just keep trying and you will crack it eventually. The vaal is one of my favourite places to fish and i wish i could get up there to fish it more. To me the Smallmouth yellowfish is one of the most awesome freshwater fish we have to target here in south africa. A true Proudly South African Icon!
Cheers
Grant.