Unfortunately, there are no clear answers to these questions. To state the obvious, no experiments have ever been performed on SN suicide. We are all just guessing what is best based on limited information from those who succeeded. To state the obvious again though, we can't ask those who succeeded how they did so.
Having said all of that, it's probably better to drink a second glass if you throw up. Generally, the advice from this website is to avoid high doses of SN due to vomiting risk. On the other hand, if you've already vomited, then this seems a moot point.