The Ran explains the tzad in the gemara that one can be sho'el on a hekem that the case would have to be where they are sho'el the same day they hear about the neder so that they will be able to be meifer after the hakama is removed, because if the day had already passed it is no different than the day passing b'shtika where the opportunity of hafara is lost עכת"ד. The Ran holds that one cannot be meifer a neder after the day he heard about it passes, even if the rationale for not being meifer on that day is because he had a hakama preventing him from being meifer. Rashash points out that Tosafos (kesubos 71b) is mesupak about this, and the Tur assumes that one can be meifer on the day they are sho'el on their hekem. Perhaps we can explain the point of argument between the Ran and Tur, to be how we view the concept of "the day he heard". The Tur understands that "the day he heard" means the day he has the opportunity and possibility of doing the hafara, so if he fails to take advantage of the opportunity until the end of the day, he looses it. But in our case where he cannot be meifer that day because he has a preexisting hakama, he would not loose his opportunity. Rather, we would look at the day he is sho'el on his hekem when the opportunity of hafara presents itself, and he fails to take advantage for that entire day, he would loose the opportunity. The Ran argues because he holds that it is not an issue failing to take advantage of the opportunity, rather he holds that as soon as he hears about it we start the clock for his ability to be meifer and that expires when the hour hits 6pm - so even if he could not be meifer on the day he heard, he would not be able to be meifer later. Another possible explanation for the Ran is that the Ran holds that if one did hakama and hafara on the day he heard of the neder, and the next day was sho'el on the hakama, the hafara would go into effect retroactively (I think R' akvia eiger says something like that). Based on this, the Ran may very well hold that he looses the opportunity of hafara because he failed to take advantage of it on the day he heard, like the Tur, but even in our case he should have taken advantage of the hafara since it would accomplish something if he were to later be sho'el on the hakama.
The Rashash asks an interesting question: Everyone seems to hold that the only type of "hakama" that one can be sho'el on is an active hakama, but if the day he heard passed and created a passive hakama, all agree that he cannot be sho'el on that hakama - Why is this true, since the whole concept of "b'yom sham'oh" is that the day passing will be a hakama, so he should be able to be sho'el on that as well? Based on the first explanation i said for the Ran that "b'yom sham'oh" is like a time clock that expires at the end of the day, it is not an issue of hakama, rather the time opportunity of hafara simply expires (but the Rashash understands based on the pasuk that it is not a time expiration, rather tantamount to an actual hakama).