We have two different nebulizers, neither are "portable" in terms of being battery operated; however, we do travel with our Pari Proneb Ultra II, which is quite compact and have used it in the car with a power inverter, though our new VW Jetta has an actual plug-in in the back seat.
The Devilbiss is a little bulkier and I wouldn't want to travel with it.
Battery operated nebulizers/Compressors like the Pari Trek are not recommended for certain medications such as Tobi or Pulmozyme. But the newer machines for Cayston I believe can be used provided the medications are compounded a certain way.
You could check out amy --- Noexuses blog for information on travelling as she's travelled extensively thru Europe, Japan using different compressors and vesting.