Vaso-vagal syncope is just a fancy way of saying that someone's blood has pooled in their lower extremities, causing them to pass out. This occurs because the blood vessels in the leg expand, pumping blood down more quickly than the heart can pump it back up into itself. It happens to roughly half of all people at some point, generally while they are young and otherwise healthy. There are all sorts of potential triggers, including pain, stress, fatigue, and dehydration.
Unless a person is having multiple episodes, it's not considered a serious condition. Passing out is draining, so the person will probably need to lie down for a couple hours. But once they rest up, they should be just fine.
Besides trying to get a full night's sleep, maintain a healthy diet, and drink lots of water, there's not a lot that can be done to reduce a person's chances of experiencing vaso-vagal syncope. Obviously, if you're in pain and have the option of not being in pain, you should go with the latter. If you're winded beyond the point of exhaustion, sit down and have a rest. However, sometimes, if you're not quick enough to realize what's happening and lie down, you're going to lose consciousness. It may help to know that vaso-vagal syncope occurs most often in the morning (so eat your Wheaties).
It varies from person to person, but I'll try to explain what it feels like with an amusing anecdote.
I am sitting at my desk at work. (I am the clerk at a motel.) I notice I'm feeling a little funny - not able to focus, losing my train of thought.
2. Sweating & Nausea
I feel sick. I haven't had breakfast, so I can safely say it isn't something I ate. All I've had in my stomach since I woke up is the nasty Farmer Brothers coffee that comprises our Continental Breakfast. I wipe cold sweat off my upper lip.
3. Shaking, Numbness
I begin shivering, not from cold (though I am clammy), but as though I'm going to puke. I feel unbalanced and betrayed as I stand up to go to the bathroom. My fingertips are numb and icy. I move as quickly as I can, sure I'm going to vomit before I get there.
4. Loss of Vision
The laundry room I have to pass through is dark. I don't notice the black clouds at the periphery of my vision until I get to the bathroom and switch on the light. I see the toilet in front of me and the shelf of cleaning utilities above it, fading.
5. Down You Go
The last thing I see is the edge of the sink as I hit my head on it, when my legs give out. It doesn't hurt.
I feel the cold, hard linoleum floor. Why am I on the floor? I was sick. I fell down. No - I passed out. I feel weak and sweaty. Using the wall, I make my way back out to the desk and press the intercom to wake the live-in manager.
If it hasn't happened to you yet, your odds are about 50/50. There's not much advice available except that, if you start feeling weak and nauseous but the need to vomit isn't imminent, you should try laying down with your feet up. Of course, your higher brain functions, like being able to think "I should put my feet up so I don't pass out", are the first to go when your blood pressure drops.
On that note, just try not to pass out.