As far as remember, the ONLY reason Maanvi gave the bloodtest the second time around is because Jeevika pretended to be sick and Dr.Manan said she needs blood but if Maanvi is scared of needles, he could arrange for someone else's blood for Jeevika.
Viren was present in the hospital at that time and he SAW how Maanvi immediately rushed to donate blood to Jeevika. He saw how she ran to the hospital with them despite hating hospitals. He heard Maanvi tell Manan that take how many ever bottles of blood you want but save my sister. I understand he was TERRIFIED seeing his wife in that condition but later, he should have suspected something. He's a very intelligent and capable lawyer.
I'm sorry but I can digest this storyline and I'm a neutral fan of the serial, the behenas and BOTH jodis. I hate when writers ruin their own characters for needless drama /: