Vomitting after any surgery is bad because it increases your pressures which can cause you to have a blood leak.
This issue should not be a huge problem, but it does have at least some potential to stir up bleeding in your wound. I would discuss with your surgeon.
Anything that leads to straining whether it be through strenuous exercise of vomiting can momentarily raise the blood pressure. These spikes in blood pressure do raise concerns of possible bleeding from recently cauterized vessels. All this being said, there is no precaution you can take other than attempting to manage your GI issues as best as you can. In all probability, you should be fine. Although it is not ideal, I would not expect it to affect your outcome especially as you get further and further from your surgery date. Hope your recovery moves forward smoothly.
Vomiting, because it potentially raises intra-abdominal pressure, can put a strain on blood vessels that have been recently cauterized. We worry about this phenomenon more in facial surgery than in breast surgery. When you have to vomit, you have to vomit and there is not a great way of stopping it. I wouldn't let it concern you too much. Congratulations on your surgery and good luck on an uneventful recovery.