There are portable SCIFs, basically specially designed trailers, to allow senior staff to communicate securely on the road. It's very likely Vance had one of these nearby.
Failing that, these people almost certainly have laptops connected to DoD networks at a lower COMSEC level than a true SCIF (indeed, "high-side laptops" were mentioned in the Signal thread). They could have communicated with those. I don't know about DoD policy if those would be acceptable or not for discussions about planned strikes, but it'd be a hell of a lot more secure than unsecured public smartphones.
There are 'tents' as well; Obama in one:
* https://www.bbc.com/news/world-us-canada-12810675
* http://archive.is/https://www.nytimes.com/2013/11/10/us/poli...