Tawassul is in fact a form of worshiping Allah and sincere Tawheed, that is why Allah (SWT) ordered the believers to do Tawassul to get forgiveness. In Sura Annisaa' verse 64: (Had they when they did injustice to themselves (committed sins) come to you then seek forgiveness and the messenger prayed for their forgiveness, they would have found Allah accepting repent and Merciful.
Allah (SWT) has ordered the believers to do Tawassul ( Sura 5, verse 35).
Tawassul does not mean worshiping other than Allah ( to be looked at as Shirk by some ignorant Wahabis) but it means seeking the blessings of the most humble servants of Allah to seek from Allah (SWT).
We believe that no wish can be granted through Tawassul but only by Allah (SWT).
In Quran we read in Sura Yousuf when his brothers requested their father (Prophet Yaqoob) to seek forgiveness from Allah for them and he accepted and said: I will seek forgiveness for you. (Sura Yousuf, verse 97). If it was wrong, the prophet Yaqoob should have told them when they did Tawassul through him.