I alternate between wafers and veggies every other day (erm one day wafers, one day veggies) but I have several plecs, so I think 2-3 times a week for veg will do. You cant feed them nothing - yep they might survive on tank muck, but it's certainly not what's best for them, so personally I'd ignore that advice (no offense!).
I give mine mainly courgette and sweet potato - as that's all they've taken a liking to so far, but apparently other folks plecs like normal potato, peas, lettuce, cucumber, carrots, melon (not tried that myself yet). You dont need to blanche it unless you have babies, just cut off a chunk/slice, wash it, weigh it down (stick a spoon through it, or a screwcumber, or some plant lead), plonk it in and then remove what's left uneaten the next morning
Some plecs dont mind the odd prawn/mussell or shrimp pellets and the like - but only as a once in a while thing - not daily.