I'm sorry to say it, but that's just the placebo effect.
Believing that swallowing mentos will make a difference, tricks your mind into thinking that it has made a difference.

Scientifically speaking, swallowing mentos makes absolutely no difference, when drinking soda.
Other than, I suppose, having the added mass of a mentos in your stomach.
Which is hardly a significant difference.
You could be eating any candy, at that point.

As for the anecdotal evidence provided, isn't it possible that your friend, who you have described as having a "smaller, weaker stomach", almost threw up because they drank too much soda, too fast?

If you'll allow me to interject with my own anecdotes; I have, by no means, a small or weak stomach. But I have made myself sick, by chugging excessive amounts of soda, sans mentos.

Again, you don't need some magic, physics-defying breath mint, to bloat up from the carbon dioxide in a bottle of soda.
It's already going to happen, from the soda alone.
So you may as well save your mentos-money, and just use it to buy more soda, instead.

There are virtually no risks with a coke and mentos bloat.
It's no more dangerous than drinking soda alone.

And there's a pretty good reason for that.

The physical reaction that takes place when you drop a mentos into a bottle of soda can not take place within the human stomach.

Said reaction, which is called "nucleation", is caused by the pitted shell of the mentos providing a place for bubbles to rapidly form, in the otherwise smooth plastic bottle.
Once you've drank the soda, the carbondioxide has already started to separate from the soda.
And swallowing a mentos does nothing to hasten that effect.

Any sensation of feeling any more bloated than you would be, from simply drinking soda, is just a placebo.

That said, there is a silver lining here.
Since the coke and mentos reaction never creates gas, and only releases what's already there, you never needed mentos anyway.
All of that carbon dioxide will still be released into your stomach.
And if you can keep yourself from burping it back up, you'll get just as bloated with soda alone as you could have hoped for from a coke and mentos reaction.

Yeah, that's a pretty average experience.
If you can manage to keep the gas down, it'll bloat you up pretty good.
But that is pretty darn tough to do.
And burping all of it up at once can be pretty harrowing.

That said, you should be safe.
A teaspoon of baking soda is well within the safe amount, and vinegar won't hurt you.
It's not pleasant, but you should be fine.

And likewise, I'd agree that there are far better ways to inflate.

Constipation, convulsions, diarrhea, frequent urination, irritability, muscle spasms, muscle weakness, vomiting, and seizures.

It's some pretty serious stuff.
So I sincerely recommend that you don't push the dosage.
And I wouldn't recommend using this method often, either.

I haven't tried mixing vinegar with anything, so I don't know how well that would work.
But as Oriktum points out, Coke is a lot easier to drink, if the vinegar isn't going down easily.
And it should work just as well, if not better.

It can be done safely.
Though, too much baking soda can cause some serious side effects.

I don't know exactly what the maximum "safe" dose would be, but a little bit goes a long way.
I'd recommend no more than 2 teaspoons.

As for how you do it, that's simple enough.
First, consume the baking soda.
I find the easiest way to do that is with water.
Either mixing it with water, or taking a swig of water, then pouring the baking soda into your mouth, and swallowing it right away, with said water.

Then, you drink the vinegar.
You don't need to drink much for the effect to take place.
A shot glass full ought to do the trick.

Alternatively, you could drink Coca-Cola, which triggers the chemical reaction just as well, with the added bonuses of carbonation, and not tasting like vinegar.
(Other brands of cola will also work, as long as you see "Phosphoric acid" in the ingredients)

I feel for you.
my room mate makes it fairly difficult to inflate in privacy.

Generally I do it in the middle of the night in the bathroom, counting on him being asleep.

I should mention however, that an electric pump wouldn't be advised for that.
Bathrooms kinda tend to act like an echo chamber.

There's also the option for oral inflation in bed by swallowing air.
It's rarely my first choice, given the whole "sleeping person in the room thing", but it hasn't caused an scene yet.

Either way, I wish you luck.
And I hope I've been of some assistance.

It only took a few weeks to be honest.

I found it helped to be in a near meditative state of calm.

Lately, I've been paying close attention to how many pumps I can get in with my hand pump, (a standard balloon animal pump attached to a tube).
It's given me a direct goal for each inflation, to beat my previous record.
Not too long ago, my max was around 100 pumps,and now I'm pushing it at around 220.

Anyways, I was wondering if anyone else likes to keep track of the number of pumps they take, and if so, what your current records are.

I've been doing this for years.
It's about time this method caught on.
And I agree with Ateitall.
It does work best laying flat with your head at a slight elevation.