Nedarim 22a - A Few quick points

1. Making a neder is building a bama - Ran explains that doing something that "feels like" a mitzvah is not necessarily a mitzvah, unless we were commanded to do it. See Beis Halevi that after the sin of the eigel, the Torah in Parshas vayakhel stresses that everything was done "as G-d commanded". The Meshech Chochma (beginning of vayikra) explains that bamos were a heter to use as an outlet for avoda zara, but not really lichatchila (like Rambam's understanding of Korbanos). Similarly one who makes a neder is trying to set a geder from an issur, but is doing an issur by adding to the Torah's list of issurim.

2. In the story where one Jew murdered another and Ulah gave him a shkoyach and then advised that he open the cut to make him die quicker - R' Yochanan says that ulah's heter was pikuach nefesh. The Rosh implies that Ulah had 2 intentions: 1. to save his life. 2. make the person die quicker (less painful). However, he was concerned that he was machzik y'dei ovrei aveirah, and to that R' Yochanan responded that for pikuach nefesh it is not a concern. It is not so clear if pikuach nefesh was necessary for Ulah's heter for expediting the person's death which is tantamount to murder. BUT, even if we assume that it was pikuach nefesh, this gemara indicates that although one cannot murder (or presumably say something that will cause murder) to save his life, that is because "who says his blood is redder". However, here where Ulah was a Chayei Olam against the victim who was a Chayei Sha'ah, we apparently are able to choose and give preference to Chayei Olam. This is really based on the Chazon Ish (Y.D. 69:2) understanding of the story with ben peturah and r' akiva [Another possibility, although a much bigger chiddush, is that even without pikuach nefesh, one may not actively b'yadayim expedite the death of another jew even to save him pain, but may advise that it be done - but let me be clear that I don't think that this is true].

3. The Ran implies that the issue of being matir a shavua that was made using the name of Hashem is not that retroactively it will turn into saying shem shamayim l'vatala, rather it is just that a shavua with the name of Hashem is very chamur and one should not be matir it. The reason that it wouldn't be shem shamayim l'vatala is because we look at the status at the time when he said it, at which point it wasn't l'vatala. This would be consistent with the Ritvah 18a that if at the time it was said it was shavuas shav, it cannot be fixed by being matir a preexisting shavua to allow this one to be chal - we always follow the status at the time that he said it and don't retroactively take it away from beign l'shav, or make it into l'shav.