Wahab has the ablity to take wickets. When Amir is not firing Wahab manages to get a few. At the moment I would take Wahab over Amir.
As for his replacement I see no one better than Junaid Khan.
Wahab has no ability to take wickets - he can buy some. What Wahab is doing is similar to Agarkar, but at much inferior impact. 15 years back, Agarkar took almost 2 wickets/ODI at 5+ economy, which was effective in one sense that he at least gave wickets with new ball for the spinners to come in to play.
On cricket tactical terms, I can explain why Amir isn't getting that many wickets in ODI - won't now, as it'll sound hollow. He is around 26, I am sure, he'll bowl sometimes in near future under Sarfraz or even himself, who knows - we'll see then; same goes for Yasir. Also, International teams don't bat with Afridi size brain, not even pea size - if they get at least couple of Hit Me ball every over from one end, they don't need to go crazy against the only bowler maintaining at least decent pressure. Show me a single match, where Amir opened the flood gate.
Besides, continuation of Misbah/Afridi style Captaincy has ensured that teams can walk around at 4.5 to 5.75 rate from over No. 11 to 40, without taking slightest of risks & can reach 180/3 to 240/2 after 40 overs, with set batsmen in - what Amir, even Wasim at prime 'll struggle to stop 100 more in 10 overs after that in current context. Bowling is the most tactical part of Cricket game, it was even 170 years back, when Alfred Mann & Fuller Pilch used to play single wicket games - in current moods operandi even Shane, Murali, Wasim, McGrath will struggle to bowl teams out if they allow "Captain" to call shots.
Junaid is a decent option; only issue is, there are too many left-armers in the team. He is not a better option than Amir or even Irfan, if fit & I'll take Tanvir in LO than Wahab.