Nedarim 9a - Shavua on Mitzvah - Ran and Rosh

I pointed out on daf 8a a machlokes between the Ran and Rosh whether a vow to fulfill a mitzvah is binding to be in violation of ba'al yachel. They seem to go li'shitasam here. They both ask the question on the mishna which implies that only reshaim make nedarim and not keshayrim, but what about a vow to fulfill a mitzvah? The Rosh says that such a vow does not count since it it not at all binding and only works as a ziruz. The Ran says that although it is binding, it doesn't fit the term "nedava" since there is already a prior obligation to keep the vow. On the previous daf, i pointed out that the Ramban quoted in the gilyon hashas seems to agree with the Rosh. However, R' Moshe (Dibros heara #51) points out that the Ketzos haChoshen (73:5) is medayek from the Ramban that although there is no korban or violation of ba'al yachel for a mitzvah vow, there is a mitzvas aseh because otherwise it would be a violation of shavuas shav. R' Moshe points out that both the Rambam and Rosh seem to hold that there is no additional Torah violation at all. His diyuk from the Rambam is that the Rambam recommends against one who makes nedarim to set them on the straight path is praiseworthy but it is preferable try and achieve the goal without the means since it may set one up for violating their vow. When it comes to a shavua to fulfill a mitzvah the Rambam does not recommend against it - R' Moshe understands that there is no additional violation, merely a ziruz and that is why there is no concern.