The VIX is designed to "represent the implied volatility of a hypothetical at-the-money [SPX] option with exactly 30 days to expiration." (via the CBOE) The calculations are available from the CBOE in this white paper.
Note that your question is wrong -- it is the implied volatility, not the vega. Moreover, you wouldn't predict a change in vega (which is a second derivative...), you'd predict the change in volatility and seek to profit through vega.
Replacing your use of vega with implied volatility, your assumption becomes correct in theory -- but it's a tough theory to put into practice... even if we suppose for a moment that you can in fact predict the VIX. It is extremely rare that you can actually purchase an ATM option with 30 days to maturity, meaning your proxy security necessarily deviates from the VIX calculation. Moreover, the transaction costs of maintaining a delta-neutral position are not insignificant (you can't just sell a put and a call, it won't stay delta-neutral).
You wouldn't do it with VIX options because they reflect the forward value of the VIX at maturity, and if you look at the term structure of VIX futures you'll see that forward values, even one month out, are far less volatile than spot.