That's strange... Are you sure you did all the other stuff already and didn't miss something? I am asking because if you did all other things then you should have a letter from Alioch after sleeping! Look around in every place and see if there is something you missed and still didn't do - may be you will find it and then be able to continue by sleeping and having the letter from Alioch!
I am not sure if that's a part of that quest with Vehlis but if you want you can try and see if it is - I am speaking about Elmy at the bar - go there and see her arguing with one of the traders orc who is selling stuff - speak with her and then Sabia will told her she will try to find how to help her - go to that orc (in the same tent from where you can buy clothing,food and other stuff and where is also the shp of Alioch) - one of the choices there will be to go speak with that orc,his name will be there too - i think was the last option of all - then speak with him and decide to help him - after Sabia deal with the orc go to Elmy again and told her about it - this way you will finish that short quest and may be then you will have the letter from Alioch on the next day if that's a part from the whole Vehlis mission too! If not then i am not sure what you are missing and why you can't have the letter from Alioch!