The Mishna categorizes a shavua to be mevatel a mitzvah as a shavuas shav, but it is not a shavu'as bituy, as the gemara explained earlier because we are already presworn from har sinai. It is clear from the mishna that not only a shavua to be mevatel a lo ta'aseh is a shavuas shav, but even a shavua not to be mekyaem a positive mitzvah such as tefillin and mezuza. We discussed earlier whether a shavua is binding on a chatzi shiur of issur. Tosafos held that a shavua to eat a chatzi shiur of issur isn't binding, but a shavua not to eat a chatzi shiur of issur would be binding. R. Akiva Eiger (Teshuvos 76) is medayek from the Rambam (shavu'os 5:3) that if one swears not to eat a half kezayis of neveila it would be binding and he would be in violation if he ate a half kezyais. But, if he would eat a full kezayis, he would only be in violation of the issur neveila but not the issur shavua. It would seem that this can only be possible if he ate the full kezayis at one time, but if he would eat a half kezayis and then another half kezayis, it is inconceivable that by eating the first half he would be in violation of the shavuah and that issur would come off by eating the second half kezayis.
R. Akiva Eiger continues that the question of the Rambam would be that since the shavua is chal on a chatzi shiur, it should also be chal for eating a full shiur using the concept fo kollel. To this he writes that a shavua b'kollel only works when you include a different min – species, but doesn't include the eating of the very same min in a different way. This yesod comes from the gemara from the fact that R. Yochanan had to establish the case of shavua being chal on issur to be kollel devarim ha'mutarim, why not say that it is chal by making a shavua not to eat assur foods and can be chal through a kollel – since the shavua is chal on the combination of different issurim, it should be chal on the full amount of issur? From the fact that R. Yochanan doesn't say this, indicates that kollel only works if it includes other minim that weren't already assur.