According to our understanding of the matter there is nothing wrong in your prayer. The matter is subject to Ijtihād and none of the two parties are violating the clear guidance of the Holy Qur’an and the Sunnah. It would have certainly caused problems if someone who believes that vomiting breaks the Wuzu led prayer after vomiting. This is not the case with the referred to Imam. Declaring your prayer invalid would certainly mean that you declare the prayers of Shafi`is invalid and that you should never offer prayer behind them. This I am sure is not the case.