Honestly, if you have a second glass ready and actually drink it if you vomit after the first glass (which is a lot easier said than done) then I think your chances of success are pretty high. Vomiting doesn't get all of it out of your system and you most likely don't need a huge amount in your system for it to be fatal.
Most of the failures arise from using a bad substance or being found/calling for help.
AE just makes the process a lot easier, would you rather be able to just take one glass, keep it down and deal with the symptoms of ~25g of SN or be put in the position where your SI is making it difficult to down a second glass and have the unease of not knowing how much SN is actually in your system, which could worsen your side effects?
But no I wouldn't say a lot of the failures are caused by a lack of AE, it just helps as a comforter. If you vomit without a second glass your chances of success are reduced though.