You won't necessarily vomit, but it is a quite common to do so when taking opiates, especially in a intolerant person.
Depending on which ROA you are using to take the Oxys vomiting may lessen your high.
-If you just eat them and you vomit, you will lose any of the drug that is not already absorbed into your body, so wherever you are in your high at that moment, is now the peak of your high and you will start to come down rapidly after that.
-If you snort the Oxy, vomiting will also lessen the high, but not as much because you will only lose whatever parts of the drug that dripped down, was swallowed after snorting, and hasn't been absorbed yet.
-Obviously if you IV, vomiting will do nothing to your high, as when you IV, all the drug is absorbed all at once and is never in your stomach, so vomiting would not rid your body of it.
As far as dosing is concerned, your friend is right, 15mg should be enough for you to get high if you don't have a tolerance. I believe Oxycodone BA via oral and intranasal administration are very similar, so the 15mg will get you just as high either way, but snorting it will help it absorb faster, so it will hit you sooner and a little bit harder.