Well, this is probably a bit late now, but the best thing to do would have been to immediately power off the MacBook (disconnect the power if it's plugged in, then remove the battery--or if it's a built-in battery, force a power-down by holding the power button for a few seconds). In any case, here are some things to try next:
- Don't turn the MacBook on for at least a week, if not longer.
- Keep the MacBook in a dry place: next to a dehumidifier, for example, in a bag of rice (although wrap the MacBook carefully in something like a towel to avoid rice or starch particles getting inside), or anywhere that might be conducive to evaporation of water. In other words, not your bathroom or your non-air-conditioned house in muggy summer weather.
- Before doing any of this, remove the battry (if you haven't already) and as many of the components under the battery cover as you can--i.e., hard drive and RAM. [Of course, you can't do this with the built-in-battery MacBooks.]
Wait patiently a week or more until you think it might be safe to try again (that does not mean five minutes after the accident

), and see what happens.
Also, you say the power light blinks twice when you try to turn it on? Some kinds of blinking indicate RAM issues, but I'm not sure if this is one of them (I believe that would be three blinks, a pause, three more blinks, a pause, and repeat in one case or a continuous fast blink). But perhaps it is a RAM issue--although in any case, I'd still wait longer, and if it doesn't work (after reinserting the RAM and HD), try removing one stick of RAM at a time to see if that helps. Or perhaps the two blinks are indicative of another issue, but I'm not familiar with what issue that pattern represents.
As for the particular substance you--er, your friend--spilled, it's definitely not as bad as sugary drinks like pop, but I'm not sure how it would compare to water, spills of which several people's machines (which may or may not include mine...

) have survived. Vinegar is normally 4-5% acetic acid diluted with water. I'd imagine the water used is distilled (i.e., mostly pure without dissolved minerals, etc.), but I'd also guess vinegar would leave behind a little acetic acid when evaporated (unless it manages to evaporate at the same time as the water)--not a lot, since the concentration in vinegar is quite low. An at room temperature, acetic acid is normally liquid as well (although below 62 it is solid). Beyond that, I haven't taken chemistry in so long that I have no further speculation.
In any case, good luck! Let us know how it turns out. I think the most important thing is to wait a while before trying again.