With Michael and Sonny both dead,and assuming that Don Corleone is wanting to retire,and hand over the reins of the Family, then Vito's options are limited.
No way,no how is Fredo going to be in charge,even as a token "front Boss".

That leaves Clemenza, Tessio, Neri,and Rocco.

Hagen, being German-Irish is totally out of the picture,but the smart move would be to keep him as the Corleone lawyer/advisor.

Neither Rocco or Neri have the experience,history with the Family,or enough respect from the other Families to be Boss,so that leaves Clem and Tess.

IMHO,Tessio has the experience,loyalty,and proven track record,as well as the ruthless streak that it would take to run things.

So Fredo still winds up getting stepped over,and maybe would have set Tess up,like he did Mike.

No matter the outcome,the thing to remember is that Barzini will be the biggest threat to the Corleone's so all decisions by Vito will be made with that in mind.

So here's my line up:

Tessio- Acting Boss until Vito's retirement or death,then Official Boss
Clemenza- Underboss
Rocco- Capo
Neri- Capo
Fredo- Capo,but limited to rackets and soldiers given to him by Vito
Hagen- legal advisor / couuselor.